Re: [patch v2 0/6] Add TRIM support for raid linear/0/1/10

[Mathias BurÃn]

On 21 March 2012 02:08, Shaohua Li <shli@xxxxxxxxxx> wrote:
> 2012/3/20 Shaohua Li <shli@xxxxxxxxxx>:
>> 2012/3/20 Holger Kiehl <Holger.Kiehl@xxxxxx>:
>>> Hello,
>>>
>>>
>>> On Tue, 20 Mar 2012, Shaohua Li wrote:
>>>
<snip>
> Thanks for the testing. The trace data is very helpful. In the intel
> SSD, trace data
> shows a discard request uses about 1 ~ 3 ms. The filesystem suffers from
> fragmentation too, so lots of small discard requests. When ext4 starts doing
> discard, it usually uses more than 1 minutes. That's too bad.
> If just looking one disk's trace data, there are some extra latencies between
> two discard requests. The combined trace data of two disks show the latency
> comes from waiting for another disk, so nothing abnormal. I thought we could
> do an optimization for this case in the future.
> So in summary, discard from the SSDs is slow. When your filesystem is
> fragmented, the performance will be terrible.
>
> Thanks,
> Shaohua
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Many people prefer to have the filesystems mounted without discard,
then they run a TRIM command/script at night (or whenever the disk
isn't used very much). This way the TRIM commands don't slow down the
'overall' performance.

Mathias
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