[LU-15829] DIO small reads fails

Mon, 31 Oct 2022 04:09:40 +0000

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]$

Whamcloud Community JIRA

[LU-15829] DIO small reads fails