Il 23-10-2019 17:37 Zdenek Kabelac ha scritto:
Hi
If you use 1MiB chunksize for thin-pool and you use 'dd' with proper
bs size
and you write 'aligned' on 1MiB boundary (be sure you user directIO,
so you are not a victim of some page cache flushing...) - there should
not be any useless read.
If you still do see such read - and you can easily reproduce this with
latest kernel - report a bug please with your reproducer and results.
Regards
Zdenek
OK, I triple-checked my numbers and you are right: on a fully updated
CentOS 7.7 x86-64 box with kernel-3.10.0-1062.4.1 and lvm2-2.02.185-2,
it seems that the behavior I observed on older (>2 years ago) is not
present anymore.
Take this original lvm setup:
[root@localhost ~]# lvs -o +chunk_size
LV VG Attr LSize Pool Origin Data% Meta% Move
Log Cpy%Sync Convert Chunk
root centos -wi-ao---- <6.20g
0
swap centos -wi-ao---- 512.00m
0
thinpool centos twi-aot--- 1.00g 25.00 14.16
64.00k
thinvol centos Vwi-a-t--- 256.00m thinpool 100.00
0
Taking a snapshot (lvcreate -s /dev/centos/thinvol -n thinsnap) and
overwriting 1 MB of data on origin via "dd if=/dev/urandom
of=/dev/centos/thinvol bs=1M count=32 oflag=direct" results in the
following I/O to/from disk:
[root@localhost ~]# dstat -d -D sdc
---dsk/sdc---
read writ
1036k 32M
As you can see, while 1 MB was indeed read (due to metadata read?), no
other read amplification occoured.
Now I got curious to see if zeroing behave in the same manner. So, I
deleted thinsnap & thinvol, toggled zeroing on (lvchange -Zy
centos/thinpool), and recreated thinvol:
[root@localhost ~]# lvs -o +chunk_size
LV VG Attr LSize Pool Origin Data% Meta% Move
Log Cpy%Sync Convert Chunk
root centos -wi-ao---- <6.20g
0
swap centos -wi-ao---- 512.00m
0
thinpool centos twi-aotz-- 1.00g 0.00 11.04
64.00k
thinvol centos Vwi-a-tz-- 256.00m thinpool 0.00
0
[root@localhost ~]# dstat -d -D sdc
--dsk/sdc--
read writ
0 13M
520k 19M
Again, no write amplificaton occoured.
Kudos to all the team for optimizing lvmthin in this manner, it really
is a flexible and great performing tool.
Regards.
--
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@xxxxxxxxxx - info@xxxxxxxxxx
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