summaryrefslogtreecommitdiff
path: root/drivers/md/dm-cache-policy-mq.c
AgeCommit message (Collapse)Author
2015-10-31dm: convert ffs to __ffsMikulas Patocka
ffs counts bit starting with 1 (for the least significant bit), __ffs counts bits starting with 0. This patch changes various occurrences of ffs to __ffs and removes subtraction of 1 from the result. Note that __ffs (unlike ffs) is not defined when called with zero argument, but it is not called with zero argument in any of these cases. Signed-off-by: Mikulas Patocka <mpatocka@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2015-08-12dm cache policy smq: move 'dm-cache-default' module alias to SMQYi Zhang
When creating dm-cache with the default policy, it will call request_module("dm-cache-default") to register the default policy. But the "dm-cache-default" alias was left referring to the MQ policy. Fix this by moving the module alias to SMQ. Fixes: bccab6a0 (dm cache: switch the "default" cache replacement policy from mq to smq) Signed-off-by: Yi Zhang <yizhan@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2015-06-17dm cache: switch the "default" cache replacement policy from mq to smqMike Snitzer
The Stochastic multiqueue (SMQ) policy (vs MQ) offers the promise of less memory utilization, improved performance and increased adaptability in the face of changing workloads. SMQ also does not have any cumbersome tuning knobs. Users may switch from "mq" to "smq" simply by appropriately reloading a DM table that is using the cache target. Doing so will cause all of the mq policy's hints to be dropped. Also, performance of the cache may degrade slightly until smq recalculates the origin device's hotspots that should be cached. In the future the "mq" policy will just silently make use of "smq" and the mq code will be removed. Signed-off-by: Mike Snitzer <snitzer@redhat.com> Acked-by: Joe Thornber <ejt@redhat.com>
2015-06-11dm cache: age and write back cache entries even without active IOJoe Thornber
The policy tick() method is normally called from interrupt context. Both the mq and smq policies do some bottom half work for the tick method in their map functions. However if no IO is going through the cache, then that bottom half work doesn't occur. With these policies this means recently hit entries do not age and do not get written back as early as we'd like. Fix this by introducing a new 'can_block' parameter to the tick() method. When this is set the bottom half work occurs immediately. 'can_block' is set when the tick method is called every second by the core target (not in interrupt context). Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2015-06-11dm cache: add fail io mode and needs_check flagJoe Thornber
If a cache metadata operation fails (e.g. transaction commit) the cache's metadata device will abort the current transaction, set a new needs_check flag, and the cache will transition to "read-only" mode. If aborting the transaction or setting the needs_check flag fails the cache will transition to "fail-io" mode. Once needs_check is set the cache device will not be allowed to activate. Activation requires write access to metadata. Future work is needed to add proper support for running the cache in read-only mode. Once in fail-io mode the cache will report a status of "Fail". Also, add commit() wrapper that will disallow commits if in read_only or fail mode. Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2015-05-29dm cache: pass a new 'critical' flag to the policies when requesting ↵Joe Thornber
writeback work We only allow non critical writeback if the origin is idle. It is up to the policy to decide what writeback work is critical. Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2015-05-29dm cache: fix race when issuing a POLICY_REPLACE operationJoe Thornber
There is a race between a policy deciding to replace a cache entry, the core target writing back any dirty data from this block, and other IO threads doing IO to the same block. This sort of problem is avoided most of the time by the core target grabbing a bio prison cell before making the request to the policy. But for a demotion the core target doesn't know which block will be demoted, so can't do this in advance. Fix this demotion race by introducing a callback to the policy interface that allows the policy to grab the cell on behalf of the core target. Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com> Cc: stable@vger.kernel.org
2015-03-31dm cache policy mq: try not to writeback data that changed in the last secondJoe Thornber
Writeback takes out a lock on the cache block, so will increase the latency for any concurrent io. This patch works by placing 2 sentinel objects on each level of the multiqueues. Every WRITEBACK_PERIOD the oldest sentinel gets moved to the newest end of the queue level. When looking for writeback work: if less than 25% of the cache is clean: we select the oldest object with the lowest hit count otherwise: we select the oldest object that is not past a writeback sentinel. Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2015-03-31dm cache policy mq: remove unused generation member of struct entryJoe Thornber
Remove to stop wasting memory. Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2015-03-31dm cache policy mq: track entries hit this 'tick' via sentinel objectsJoe Thornber
A sentinel object is placed on each level of the multiqueues. When an object is hit it is requeued behind the sentinel. When the tick is incremented we iterate through all objects behind the sentinel and update the hit_count, then reposition the sentinel at the very back. This saves memory by avoiding tracking the tick explicitly for every struct entry object in the multiqueues. Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2015-03-31dm cache policy mq: remove queue_shift_down()Joe Thornber
queue_shift_down() didn't adjust the hit_counts to the new levels, so it just had the effect of scrambling levels. Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2015-03-31dm cache policy mq: keep track of the number of entries in a multiqueueJoe Thornber
Small optimisation, now queue_empty() doesn't need to walk all levels of the multiqueue. Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2014-11-10dm cache policy mq: simplify ability to promote sequential IO to the cacheMike Snitzer
Before, if the user wanted sequential IO to be promoted to the cache they'd have to set sequential_threshold to some nebulous large value. Now, the user may easily disable sequential IO detection (and sequential IO's implicit bypass of the cache) by setting sequential_threshold to 0. Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2014-11-10dm cache policy mq: tweak algorithm that decides when to promote a blockJoe Thornber
Rather than maintaining a separate promote_threshold variable that we periodically update we now use the hit count of the oldest clean block. Also add a fudge factor to discourage demoting dirty blocks. With some tests this has a sizeable difference, because the old code was too eager to demote blocks. For example, device-mapper-test-suite's git_extract_cache_quick test goes from taking 190 seconds, to 142 (linear on spindle takes 250). Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2014-02-28dm cache mq: fix memory allocation failure for large cache devicesHeinz Mauelshagen
The memory allocated for the multiqueue policy's hash table doesn't need to be physically contiguous. Use vzalloc() instead of kzalloc(). Fedora has been carrying this fix since 10/10/2013. Failure seen during creation of a 10TB cached device with a 2048 sector block size and 411GB cache size: dmsetup: page allocation failure: order:9, mode:0x10c0d0 CPU: 11 PID: 29235 Comm: dmsetup Not tainted 3.10.4 #3 Hardware name: Supermicro X8DTL/X8DTL, BIOS 2.1a 12/30/2011 000000000010c0d0 ffff880090941898 ffffffff81387ab4 ffff880090941928 ffffffff810bb26f 0000000000000009 000000000010c0d0 ffff880090941928 ffffffff81385dbc ffffffff815f3840 ffffffff00000000 000002000010c0d0 Call Trace: [<ffffffff81387ab4>] dump_stack+0x19/0x1b [<ffffffff810bb26f>] warn_alloc_failed+0x110/0x124 [<ffffffff81385dbc>] ? __alloc_pages_direct_compact+0x17c/0x18e [<ffffffff810bda2e>] __alloc_pages_nodemask+0x6c7/0x75e [<ffffffff810bdad7>] __get_free_pages+0x12/0x3f [<ffffffff810ea148>] kmalloc_order_trace+0x29/0x88 [<ffffffff810ec1fd>] __kmalloc+0x36/0x11b [<ffffffffa031eeed>] ? mq_create+0x1dc/0x2cf [dm_cache_mq] [<ffffffffa031efc0>] mq_create+0x2af/0x2cf [dm_cache_mq] [<ffffffffa0314605>] dm_cache_policy_create+0xa7/0xd2 [dm_cache] [<ffffffffa0312530>] ? cache_ctr+0x245/0xa13 [dm_cache] [<ffffffffa031263e>] cache_ctr+0x353/0xa13 [dm_cache] [<ffffffffa012b916>] dm_table_add_target+0x227/0x2ce [dm_mod] [<ffffffffa012e8e4>] table_load+0x286/0x2ac [dm_mod] [<ffffffffa012e65e>] ? dev_wait+0x8a/0x8a [dm_mod] [<ffffffffa012e324>] ctl_ioctl+0x39a/0x3c2 [dm_mod] [<ffffffffa012e35a>] dm_ctl_ioctl+0xe/0x12 [dm_mod] [<ffffffff81101181>] vfs_ioctl+0x21/0x34 [<ffffffff811019d3>] do_vfs_ioctl+0x3b1/0x3f4 [<ffffffff810f4d2e>] ? ____fput+0x9/0xb [<ffffffff81050b6c>] ? task_work_run+0x7e/0x92 [<ffffffff81101a68>] SyS_ioctl+0x52/0x82 [<ffffffff81391d92>] system_call_fastpath+0x16/0x1b Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com> Cc: stable@vger.kernel.org
2014-01-30Merge branch 'for-3.14/core' of git://git.kernel.dk/linux-blockLinus Torvalds
Pull core block IO changes from Jens Axboe: "The major piece in here is the immutable bio_ve series from Kent, the rest is fairly minor. It was supposed to go in last round, but various issues pushed it to this release instead. The pull request contains: - Various smaller blk-mq fixes from different folks. Nothing major here, just minor fixes and cleanups. - Fix for a memory leak in the error path in the block ioctl code from Christian Engelmayer. - Header export fix from CaiZhiyong. - Finally the immutable biovec changes from Kent Overstreet. This enables some nice future work on making arbitrarily sized bios possible, and splitting more efficient. Related fixes to immutable bio_vecs: - dm-cache immutable fixup from Mike Snitzer. - btrfs immutable fixup from Muthu Kumar. - bio-integrity fix from Nic Bellinger, which is also going to stable" * 'for-3.14/core' of git://git.kernel.dk/linux-block: (44 commits) xtensa: fixup simdisk driver to work with immutable bio_vecs block/blk-mq-cpu.c: use hotcpu_notifier() blk-mq: for_each_* macro correctness block: Fix memory leak in rw_copy_check_uvector() handling bio-integrity: Fix bio_integrity_verify segment start bug block: remove unrelated header files and export symbol blk-mq: uses page->list incorrectly blk-mq: use __smp_call_function_single directly btrfs: fix missing increment of bi_remaining Revert "block: Warn and free bio if bi_end_io is not set" block: Warn and free bio if bi_end_io is not set blk-mq: fix initializing request's start time block: blk-mq: don't export blk_mq_free_queue() block: blk-mq: make blk_sync_queue support mq block: blk-mq: support draining mq queue dm cache: increment bi_remaining when bi_end_io is restored block: fixup for generic bio chaining block: Really silence spurious compiler warnings block: Silence spurious compiler warnings block: Kill bio_pair_split() ...
2014-01-16dm cache: add policy name to status outputMike Snitzer
The cache's policy may have been established using the "default" alias, which is currently the "mq" policy but the default policy may change in the future. It is useful to know exactly which policy is being used. Add a 'real' member to the dm_cache_policy_type structure and have the "default" dm_cache_policy_type point to the real "mq" dm_cache_policy_type. Update dm_cache_policy_get_name() to check if real is set, if so report the name of the real policy (not the alias). Requested-by: Jonathan Brassow <jbrassow@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2014-01-07dm cache policy mq: introduce three promotion threshold tunablesJoe Thornber
Internally the mq policy maintains a promotion threshold variable. If the hit count of a block not in the cache goes above this threshold it gets promoted to the cache. This patch introduces three new tunables that allow you to tweak the promotion threshold by adding a small value. These adjustments depend on the io type: read_promote_adjustment: READ io, default 4 write_promote_adjustment: WRITE io, default 8 discard_promote_adjustment: READ/WRITE io to a discarded block, default 1 If you're trying to quickly warm a new cache device you may wish to reduce these to encourage promotion. Remember to switch them back to their defaults after the cache fills though. Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2014-01-07dm cache policy mq: use list_del_init instead of list_del + INIT_LIST_HEADWei Yongjun
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn> Acked-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2013-12-31Merge tag 'v3.13-rc6' into for-3.14/coreJens Axboe
Needed to bring blk-mq uptodate, since changes have been going in since for-3.14/core was established. Fixup merge issues related to the immutable biovec changes. Signed-off-by: Jens Axboe <axboe@kernel.dk> Conflicts: block/blk-flush.c fs/btrfs/check-integrity.c fs/btrfs/extent_io.c fs/btrfs/scrub.c fs/logfs/dev_bdev.c
2013-12-10dm cache policy mq: fix promotions to occur as expectedJoe Thornber
Micro benchmarks that repeatedly issued IO to a single block were failing to cause a promotion from the origin device to the cache. Fix this by not updating the stats during map() if -EWOULDBLOCK will be returned. The mq policy will only update stats, consider migration, etc, once per tick period (a unit of time established between dm-cache core and the policies). When the IO thread calls the policy's map method, if it would like to migrate the associated block it returns -EWOULDBLOCK, the IO then gets handed over to a worker thread which handles the migration. The worker thread calls map again, to check the migration is still needed (avoids a race among other things). *BUT*, before this fix, if we were still in the same tick period the stats were already updated by the previous map call -- so the migration would no longer be requested. Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2013-11-23block: Abstract out bvec iteratorKent Overstreet
Immutable biovecs are going to require an explicit iterator. To implement immutable bvecs, a later patch is going to add a bi_bvec_done member to this struct; for now, this patch effectively just renames things. Signed-off-by: Kent Overstreet <kmo@daterainc.com> Cc: Jens Axboe <axboe@kernel.dk> Cc: Geert Uytterhoeven <geert@linux-m68k.org> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> Cc: Paul Mackerras <paulus@samba.org> Cc: "Ed L. Cashin" <ecashin@coraid.com> Cc: Nick Piggin <npiggin@kernel.dk> Cc: Lars Ellenberg <drbd-dev@lists.linbit.com> Cc: Jiri Kosina <jkosina@suse.cz> Cc: Matthew Wilcox <willy@linux.intel.com> Cc: Geoff Levand <geoff@infradead.org> Cc: Yehuda Sadeh <yehuda@inktank.com> Cc: Sage Weil <sage@inktank.com> Cc: Alex Elder <elder@inktank.com> Cc: ceph-devel@vger.kernel.org Cc: Joshua Morris <josh.h.morris@us.ibm.com> Cc: Philip Kelleher <pjk1939@linux.vnet.ibm.com> Cc: Rusty Russell <rusty@rustcorp.com.au> Cc: "Michael S. Tsirkin" <mst@redhat.com> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> Cc: Jeremy Fitzhardinge <jeremy@goop.org> Cc: Neil Brown <neilb@suse.de> Cc: Alasdair Kergon <agk@redhat.com> Cc: Mike Snitzer <snitzer@redhat.com> Cc: dm-devel@redhat.com Cc: Martin Schwidefsky <schwidefsky@de.ibm.com> Cc: Heiko Carstens <heiko.carstens@de.ibm.com> Cc: linux390@de.ibm.com Cc: Boaz Harrosh <bharrosh@panasas.com> Cc: Benny Halevy <bhalevy@tonian.com> Cc: "James E.J. Bottomley" <JBottomley@parallels.com> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: "Nicholas A. Bellinger" <nab@linux-iscsi.org> Cc: Alexander Viro <viro@zeniv.linux.org.uk> Cc: Chris Mason <chris.mason@fusionio.com> Cc: "Theodore Ts'o" <tytso@mit.edu> Cc: Andreas Dilger <adilger.kernel@dilger.ca> Cc: Jaegeuk Kim <jaegeuk.kim@samsung.com> Cc: Steven Whitehouse <swhiteho@redhat.com> Cc: Dave Kleikamp <shaggy@kernel.org> Cc: Joern Engel <joern@logfs.org> Cc: Prasad Joshi <prasadjoshi.linux@gmail.com> Cc: Trond Myklebust <Trond.Myklebust@netapp.com> Cc: KONISHI Ryusuke <konishi.ryusuke@lab.ntt.co.jp> Cc: Mark Fasheh <mfasheh@suse.com> Cc: Joel Becker <jlbec@evilplan.org> Cc: Ben Myers <bpm@sgi.com> Cc: xfs@oss.sgi.com Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Frederic Weisbecker <fweisbec@gmail.com> Cc: Ingo Molnar <mingo@redhat.com> Cc: Len Brown <len.brown@intel.com> Cc: Pavel Machek <pavel@ucw.cz> Cc: "Rafael J. Wysocki" <rjw@sisk.pl> Cc: Herton Ronaldo Krzesinski <herton.krzesinski@canonical.com> Cc: Ben Hutchings <ben@decadent.org.uk> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Guo Chao <yan@linux.vnet.ibm.com> Cc: Tejun Heo <tj@kernel.org> Cc: Asai Thambi S P <asamymuthupa@micron.com> Cc: Selvan Mani <smani@micron.com> Cc: Sam Bradshaw <sbradshaw@micron.com> Cc: Wei Yongjun <yongjun_wei@trendmicro.com.cn> Cc: "Roger Pau Monné" <roger.pau@citrix.com> Cc: Jan Beulich <jbeulich@suse.com> Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com> Cc: Ian Campbell <Ian.Campbell@citrix.com> Cc: Sebastian Ott <sebott@linux.vnet.ibm.com> Cc: Christian Borntraeger <borntraeger@de.ibm.com> Cc: Minchan Kim <minchan@kernel.org> Cc: Jiang Liu <jiang.liu@huawei.com> Cc: Nitin Gupta <ngupta@vflare.org> Cc: Jerome Marchand <jmarchand@redhat.com> Cc: Joe Perches <joe@perches.com> Cc: Peng Tao <tao.peng@emc.com> Cc: Andy Adamson <andros@netapp.com> Cc: fanchaoting <fanchaoting@cn.fujitsu.com> Cc: Jie Liu <jeff.liu@oracle.com> Cc: Sunil Mushran <sunil.mushran@gmail.com> Cc: "Martin K. Petersen" <martin.petersen@oracle.com> Cc: Namjae Jeon <namjae.jeon@samsung.com> Cc: Pankaj Kumar <pankaj.km@samsung.com> Cc: Dan Magenheimer <dan.magenheimer@oracle.com> Cc: Mel Gorman <mgorman@suse.de>6
2013-11-12dm cache: resolve small nits and improve DocumentationMike Snitzer
Document passthrough mode, cache shrinking, and cache invalidation. Also, use strcasecmp() and hlist_unhashed(). Reported-by: Alasdair G Kergon <agk@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2013-11-11dm cache: add remove_cblock method to policy interfaceJoe Thornber
Implement policy_remove_cblock() and add remove_cblock method to the mq policy. These methods will be used by the following cache block invalidation patch which adds the 'invalidate_cblocks' message to the cache core. Also, update some comments in dm-cache-policy.h Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2013-11-11dm cache policy mq: reduce memory requirementsJoe Thornber
Rather than storing the cblock in each cache entry, we allocate all entries in an array and infer the cblock from the entry position. Saves 4 bytes of memory per cache block. In addition, this gives us an easy way of looking up cache entries by cblock. We no longer need to keep an explicit bitset to track which cblocks have been allocated. And no searching is needed to find free cblocks. Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2013-11-09dm cache: be much more aggressive about promoting writes to discarded blocksJoe Thornber
Previously these promotions only got priority if there were unused cache blocks. Now we give them priority if there are any clean blocks in the cache. The fio_soak_test in the device-mapper-test-suite now gives uniform performance across subvolumes (~16 seconds). Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2013-11-09dm cache policy mq: implement writeback_work() and mq_{set,clear}_dirty()Joe Thornber
There are now two multiqueues for in cache blocks. A clean one and a dirty one. writeback_work comes from the dirty one. Demotions come from the clean one. There are two benefits: - Performance improvement, since demoting a clean block is a noop. - The cache cleans itself when io load is light. Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2013-11-09dm cache policy mq: a few small fixesJoe Thornber
Rename takeout_queue to concat_queue. Fix a harmless bug in mq policies pop() function. Currently pop() always succeeds, with up coming changes this wont be the case. Fix typo in comment above pre_cache_to_cache prototype. Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2013-11-09dm cache policy mq: protect residency method with existing mutexJoe Thornber
It is safe to use a mutex in mq_residency() at this point since it is only called from ioctl context. But future-proof mq_residency() by using might_sleep() to catch new contexts that cannot sleep. Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2013-08-16dm cache: avoid conflicting remove_mapping() in mq policyGeert Uytterhoeven
On sparc32, which includes <linux/swap.h> from <asm/pgtable_32.h>: drivers/md/dm-cache-policy-mq.c:962:13: error: conflicting types for 'remove_mapping' include/linux/swap.h:285:12: note: previous declaration of 'remove_mapping' was here As mq_remove_mapping() already exists, and the local remove_mapping() is used only once, inline it manually to avoid the conflict. Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org> Signed-off-by: Mike Snitzer <snitzer@redhat.com> Signed-off-by: Alasdair Kergon <agk@redhat.com> Acked-by: Joe Thornber <ejt@redhat.com>
2013-03-20dm cache: policy change version from string to integer setMike Snitzer
Separate dm cache policy version string into 3 unsigned numbers corresponding to major, minor and patchlevel and store them at the end of the on-disk metadata so we know which version of the policy generated the hints in case a future version wants to use them differently. Signed-off-by: Mike Snitzer <snitzer@redhat.com> Signed-off-by: Alasdair G Kergon <agk@redhat.com>
2013-03-01dm cache: add mq policyJoe Thornber
A cache policy that uses a multiqueue ordered by recent hit count to select which blocks should be promoted and demoted. This is meant to be a general purpose policy. It prioritises reads over writes. Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Alasdair G Kergon <agk@redhat.com>