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authorDave Chinner <dchinner@redhat.com>2013-02-11 16:05:01 +1100
committerBen Myers <bpm@sgi.com>2013-02-14 17:21:32 -0600
commita1e16c26660b301cc8423185924cf1b0b16ea92b (patch)
treeb0dfc8be0dd7a91d32b3280517ca64ca3f3c5f79
parent311f08acde635e4e5ccea9b9d8c856cc2e0ced95 (diff)
xfs: limit speculative prealloc size on sparse files
Speculative preallocation based on the current file size works well for contiguous files, but is sub-optimal for sparse files where the EOF preallocation can fill holes and result in large amounts of zeros being written when it is not necessary. The algorithm is modified to prevent EOF speculative preallocation from triggering larger allocations on IO patterns of truncate--to-zero-seek-write-seek-write-.... which results in non-sparse files for large files. This, unfortunately, is the way cp now behaves when copying sparse files and so needs to be fixed. What this code does is that it looks at the existing extent adjacent to the current EOF and if it determines that it is a hole we disable speculative preallocation altogether. To avoid the next write from doing a large prealloc, it takes the size of subsequent preallocations from the current size of the existing EOF extent. IOWs, if you leave a hole in the file, it resets preallocation behaviour to the same as if it was a zero size file. Example new behaviour: $ xfs_io -f -c "pwrite 0 31m" \ -c "pwrite 33m 1m" \ -c "pwrite 128m 1m" \ -c "fiemap -v" /mnt/scratch/blah wrote 32505856/32505856 bytes at offset 0 31 MiB, 7936 ops; 0.0000 sec (1.608 GiB/sec and 421432.7439 ops/sec) wrote 1048576/1048576 bytes at offset 34603008 1 MiB, 256 ops; 0.0000 sec (1.462 GiB/sec and 383233.5329 ops/sec) wrote 1048576/1048576 bytes at offset 134217728 1 MiB, 256 ops; 0.0000 sec (1.719 GiB/sec and 450704.2254 ops/sec) /mnt/scratch/blah: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..65535]: 96..65631 65536 0x0 1: [65536..67583]: hole 2048 2: [67584..69631]: 67680..69727 2048 0x0 3: [69632..262143]: hole 192512 4: [262144..264191]: 262240..264287 2048 0x1 Signed-off-by: Dave Chinner <dchinner@redhat.com> Reviewed-by: Mark Tinguely <tinguely@sgi.com> Reviewed-by: Brian Foster <bfoster@redhat.com> Signed-off-by: Ben Myers <bpm@sgi.com>
-rw-r--r--fs/xfs/xfs_iomap.c77
1 files changed, 67 insertions, 10 deletions
diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 364818eef40e..912d83d8860a 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -311,6 +311,62 @@ xfs_iomap_eof_want_preallocate(
}
/*
+ * Determine the initial size of the preallocation. We are beyond the current
+ * EOF here, but we need to take into account whether this is a sparse write or
+ * an extending write when determining the preallocation size. Hence we need to
+ * look up the extent that ends at the current write offset and use the result
+ * to determine the preallocation size.
+ *
+ * If the extent is a hole, then preallocation is essentially disabled.
+ * Otherwise we take the size of the preceeding data extent as the basis for the
+ * preallocation size. If the size of the extent is greater than half the
+ * maximum extent length, then use the current offset as the basis. This ensures
+ * that for large files the preallocation size always extends to MAXEXTLEN
+ * rather than falling short due to things like stripe unit/width alignment of
+ * real extents.
+ */
+STATIC int
+xfs_iomap_eof_prealloc_initial_size(
+ struct xfs_mount *mp,
+ struct xfs_inode *ip,
+ xfs_off_t offset,
+ xfs_bmbt_irec_t *imap,
+ int nimaps)
+{
+ xfs_fileoff_t start_fsb;
+ int imaps = 1;
+ int error;
+
+ ASSERT(nimaps >= imaps);
+
+ /* if we are using a specific prealloc size, return now */
+ if (mp->m_flags & XFS_MOUNT_DFLT_IOSIZE)
+ return 0;
+
+ /*
+ * As we write multiple pages, the offset will always align to the
+ * start of a page and hence point to a hole at EOF. i.e. if the size is
+ * 4096 bytes, we only have one block at FSB 0, but XFS_B_TO_FSB(4096)
+ * will return FSB 1. Hence if there are blocks in the file, we want to
+ * point to the block prior to the EOF block and not the hole that maps
+ * directly at @offset.
+ */
+ start_fsb = XFS_B_TO_FSB(mp, offset);
+ if (start_fsb)
+ start_fsb--;
+ error = xfs_bmapi_read(ip, start_fsb, 1, imap, &imaps, XFS_BMAPI_ENTIRE);
+ if (error)
+ return 0;
+
+ ASSERT(imaps == 1);
+ if (imap[0].br_startblock == HOLESTARTBLOCK)
+ return 0;
+ if (imap[0].br_blockcount <= (MAXEXTLEN >> 1))
+ return imap[0].br_blockcount;
+ return XFS_B_TO_FSB(mp, offset);
+}
+
+/*
* If we don't have a user specified preallocation size, dynamically increase
* the preallocation size as the size of the file grows. Cap the maximum size
* at a single extent or less if the filesystem is near full. The closer the
@@ -319,20 +375,19 @@ xfs_iomap_eof_want_preallocate(
STATIC xfs_fsblock_t
xfs_iomap_prealloc_size(
struct xfs_mount *mp,
- struct xfs_inode *ip)
+ struct xfs_inode *ip,
+ xfs_off_t offset,
+ struct xfs_bmbt_irec *imap,
+ int nimaps)
{
xfs_fsblock_t alloc_blocks = 0;
- if (!(mp->m_flags & XFS_MOUNT_DFLT_IOSIZE)) {
+ alloc_blocks = xfs_iomap_eof_prealloc_initial_size(mp, ip, offset,
+ imap, nimaps);
+ if (alloc_blocks > 0) {
int shift = 0;
int64_t freesp;
- /*
- * rounddown_pow_of_two() returns an undefined result
- * if we pass in alloc_blocks = 0. Hence the "+ 1" to
- * ensure we always pass in a non-zero value.
- */
- alloc_blocks = XFS_B_TO_FSB(mp, XFS_ISIZE(ip)) + 1;
alloc_blocks = XFS_FILEOFF_MIN(MAXEXTLEN,
rounddown_pow_of_two(alloc_blocks));
@@ -399,7 +454,6 @@ xfs_iomap_write_delay(
extsz = xfs_get_extsz_hint(ip);
offset_fsb = XFS_B_TO_FSBT(mp, offset);
-
error = xfs_iomap_eof_want_preallocate(mp, ip, offset, count,
imap, XFS_WRITE_IMAPS, &prealloc);
if (error)
@@ -407,7 +461,10 @@ xfs_iomap_write_delay(
retry:
if (prealloc) {
- xfs_fsblock_t alloc_blocks = xfs_iomap_prealloc_size(mp, ip);
+ xfs_fsblock_t alloc_blocks;
+
+ alloc_blocks = xfs_iomap_prealloc_size(mp, ip, offset, imap,
+ XFS_WRITE_IMAPS);
aligned_offset = XFS_WRITEIO_ALIGN(mp, (offset + count - 1));
ioalign = XFS_B_TO_FSBT(mp, aligned_offset);