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authorXavi Hernandez <xhernandez@redhat.com>2019-07-17 14:50:22 +0200
committerPranith Kumar Karampuri <pkarampu@redhat.com>2019-07-24 10:20:08 +0000
commitb01a43586c5abc23a874e5528a063c508f952cbd (patch)
tree18ed2a4d27758b069f8f44451b797c6b4ac40fb2 /tests/basic
parentdafab9fd5e4afe0f1cdd8d2e3ce7b4037faf4830 (diff)
cluster/ec: fix EIO error for concurrent writes on sparse files
EC doesn't allow concurrent writes on overlapping areas, they are serialized. However non-overlapping writes are serviced in parallel. When a write is not aligned, EC first needs to read the entire chunk from disk, apply the modified fragment and write it again. The problem appears on sparse files because a write to an offset implicitly creates data on offsets below it (so, in some way, they are overlapping). For example, if a file is empty and we read 10 bytes from offset 10, read() will return 0 bytes. Now, if we write one byte at offset 1M and retry the same read, the system call will return 10 bytes (all containing 0's). So if we have two writes, the first one at offset 10 and the second one at offset 1M, EC will send both in parallel because they do not overlap. However, the first one will try to read missing data from the first chunk (i.e. offsets 0 to 9) to recombine the entire chunk and do the final write. This read will happen in parallel with the write to 1M. What could happen is that half of the bricks process the write before the read, and the half do the read before the write. Some bricks will return 10 bytes of data while the otherw will return 0 bytes (because the file on the brick has not been expanded yet). When EC tries to recombine the answers from the bricks, it can't, because it needs more than half consistent answers to recover the data. So this read fails with EIO error. This error is propagated to the parent write, which is aborted and EIO is returned to the application. The issue happened because EC assumed that a write to a given offset implies that offsets below it exist. This fix prevents the read of the chunk from bricks if the current size of the file is smaller than the read chunk offset. This size is correctly tracked, so this fixes the issue. Also modifying ec-stripe.t file for Test #13 within it. In this patch, if a file size is less than the offset we are writing, we fill zeros in head and tail and do not consider it strip cache miss. That actually make sense as we know what data that part holds and there is no need of reading it from bricks. Change-Id: Ic342e8c35c555b8534109e9314c9a0710b6225d6 Fixes: bz#1730715 Signed-off-by: Xavi Hernandez <xhernandez@redhat.com>
Diffstat (limited to 'tests/basic')
-rw-r--r--tests/basic/ec/ec-stripe.t2
1 files changed, 1 insertions, 1 deletions
diff --git a/tests/basic/ec/ec-stripe.t b/tests/basic/ec/ec-stripe.t
index 1e940eb..98b9229 100644
--- a/tests/basic/ec/ec-stripe.t
+++ b/tests/basic/ec/ec-stripe.t
@@ -202,7 +202,7 @@ TEST truncate -s 0 $B0/test_file
TEST truncate -s 0 $M0/test_file
TEST dd if=$B0/misc_file of=$B0/test_file bs=1022 count=5 oflag=seek_bytes,sync seek=400 conv=notrunc
TEST dd if=$B0/misc_file of=$M0/test_file bs=1022 count=5 oflag=seek_bytes,sync seek=400 conv=notrunc
-check_statedump_md5sum 4 5
+check_statedump_md5sum 4 4
clean_file_unmount
### 14 - Truncate to invalidate all but one the stripe in cache ####