[linux-lvm] exposing snapshot block device

[Gionatan Danti]

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 at 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 at 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 at 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 at 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 at assyoma.it - info at assyoma.it
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