[linux-lvm] Very slow i/o after snapshotting

[Micky]

Read/write speed is just fine (~100-200MB/s) as I described in my
first few emails without LVM snapshots.


On Tue, Jul 9, 2013 at 11:47 PM, Zdenek Kabelac <zkabelac at redhat.com> wrote:
> Dne 9.7.2013 17:39, Micky napsal(a):
>
>> I meant alignment for all dm entries 0 through 31 is zero!
>>
>>
>>>> What does the `lsblk -t` say? Could be an alignment issue.
>>>
>>>
>>> 0 through 31
>>>
>>>> What's `free` saying about the free memory and cache? (dmeventd on 6.4
>>>> is
>>>> trying to lock a large chunk of address space in RAM (~100M)
>>>
>>>
>>> Cached mem looks good.
>>> Dmeventd. Right. It is. Isn't it spawed everytime an LV is created?
>>> root      6813  0.0  1.4 197056 11044 ?        S<s  May26   2:44
>>> /sbin/dmeventd
>>
>>
>
> There is only one dmeventd running - and lvm is spawning it only when it's
> not
> available - and in fact spawning is not the right term if you use systemd
> enabled system (like Fedora)
>
> Also so far you still have not show actually any 'real' numbers even when
> you run plain good old  'dd' command.
>
> So what is the performance of 'dd'  reading 10G >/dev/null
> or raw device,  dm origin, dm snapshot  (with  iflag=direct)
>
> What is performance of write ?
>
> What is the performance when 2 of them are running in parallel.
>
> Also it's probably more easier to resolve this through #irc.
>
> Zdenek
>
>