[sheepdog-users] Different write speed between vdi

[Valerio Pachera]

After I've been taking the snapshot of vdi 'backup', write speed went
from 15 to 6/7 M/s and it doesn't change even if I rewrite multiple
times a file.
I also tried to remove the snapshot and rewrite the same file several
time and the speed is still 6/7 M/s (except once).
Am I misunderstanding 'only first write will be slowed down'?

With the snapshot

2013/8/12 Valerio Pachera <sirio81 at gmail.com>:
> root at backup:~# dd if=/dev/zero of=/tmp/c2 bs=1M count=512 oflag=direct
> 536870912 byte (537 MB) copiati, 79,5951 s, 6,7 MB/s
>
> root at backup:~# dd if=/dev/zero of=/tmp/c2 bs=1M count=512 oflag=direct
> 536870912 byte (537 MB) copiati, 75,3883 s, 7,1 MB/s
>
> root at backup:~# dd if=/dev/zero of=/tmp/c2 bs=1M count=512 oflag=direct
> 536870912 byte (537 MB) copiati, 70,908 s, 7,6 MB/s


After removing the snapshot

dd if=/dev/zero of=/tmp/c2 bs=1M count=128 oflag=direct
134217728 byte (134 MB) copiati, 19,9556 s, 6,7 MB/s

dd if=/dev/zero of=/tmp/c2 bs=1M count=128 oflag=direct
134217728 byte (134 MB) copiati, 19,2722 s, 7,0 MB/s

dd if=/dev/zero of=/tmp/c2 bs=1M count=128 oflag=direct
134217728 byte (134 MB) copiati, 10,3443 s, 13,0 MB/s

dd if=/dev/zero of=/tmp/c2 bs=1M count=128 oflag=direct
134217728 byte (134 MB) copiati, 18,3039 s, 7,3 MB/s

dd if=/dev/zero of=/tmp/c2 bs=1M count=128 oflag=direct
134217728 byte (134 MB) copiati, 18,63 s, 7,2 MB/s

dd if=/dev/zero of=/tmp/c2 bs=1M count=128 oflag=direct
134217728 byte (134 MB) copiati, 19,2722 s, 7,6 MB/s