[LU-15829] DIO small reads fails

• Type: Bug
• Resolution: Fixed
• Priority: Major
• Fix Version/s: Lustre 2.16.0
• Affects Version/s: Lustre 2.15.0
• Severity: 3
• Assignee: Shaun Tancheff
• Reporter: Shaun Tancheff

Description

On Lustre there is one strange quirk:

DIO writing now works, assuming you are writing from an aligned buffer, you are not restricted to the 512 byte limit.

However DIO reading of a single byte is broken, reading for more than 1 byte is okay.

Attached test source: dio_test.c

[shaun@alma8 dio]$ /home/shaun/tools/dio/dio_test -w -d -b 1 /mnt/lustre/tmp/dio_w3.dat
/mnt/lustre/tmp/dio_w3.dat - DIRECT WRITE 1/1 bytes
[shaun@alma8 dio]$ /home/shaun/tools/dio/dio_test -d -b 1 /mnt/lustre/tmp/dio_w3.dat
read: 0 (EOF)

If the block you are reading 1 byte from is in the PAGE_CACHE then the 1 byte read will succeed.

[shaun@alma8 dio]$ /home/shaun/tools/dio/dio_test -b 1 /mnt/lustre/tmp/dio_w3.dat
read: 1, x'33  '
/mnt/lustre/tmp/dio_w3.dat - BUFFER READ 1/1 bytes
[shaun@alma8 dio]$ /home/shaun/tools/dio/dio_test -d -b 1 /mnt/lustre/tmp/dio_w3.dat
read: 1, x'33  '
/mnt/lustre/tmp/dio_w3.dat - DIRECT READ 1/1 bytes
[shaun@alma8 dio]$ echo 3 | sudo tee /proc/sys/vm/drop_caches
3
[shaun@alma8 dio]$ /home/shaun/tools/dio/dio_test -d -b 1 /mnt/lustre/tmp/dio_w3.dat
read: 0 (EOF)

So what happens when doing a DIO read of 2 or more bytes and the block is NOT in the PAGE_CACHE?

[shaun@alma8 dio]$ echo 3 | sudo tee /proc/sys/vm/drop_caches
3
[shaun@alma8 dio]$ /home/shaun/tools/dio/dio_test -d -b 1 /mnt/lustre/tmp/dio_w3.dat
read: 0 (EOF)
[shaun@alma8 dio]$ /home/shaun/tools/dio/dio_test -d -b 2 /mnt/lustre/tmp/dio_w3.dat
read: 1, x'33  '
read: 0 (EOF)
/mnt/lustre/tmp/dio_w3.dat - DIRECT READ 1/2 bytes
[shaun@alma8 dio]$ 

Comments

[Peter Jones]

Landed for 2.16

[Gerrit Updater]

"Alexey Lyashkov <[e-mail]>" uploaded a new patch: https://review.whamcloud.com/47395
Subject: LU-15829 llite: don't use a kms if it invalid.
Project: fs/lustre-release
Branch: master
Current Patch Set: 1
Commit: b96430385b93e30f0bea1e147e0b17f28577bdcd

[Gerrit Updater]

"Shaun Tancheff <[e-mail]>" uploaded a new patch: https://review.whamcloud.com/47225
Subject: LU-15829 clio: Force buffered I/O on small reads
Project: fs/lustre-release
Branch: master
Current Patch Set: 1
Commit: 57f500ecaada808af3a2fb5f5dd5d895d5a6aa3b