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Date: Mon, 06 Jan 2014 23:07:17 +0100
From: Miroslav Lachman <000.fbsd@quip.cz>
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To: Karl Pielorz <kpielorz_lst@tdx.co.uk>
Subject: Re: Is TRIM working with gmirror?
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Karl Pielorz wrote:
[...]
>> WARNING: /ssd_db: TRIM flag on fs but disk does not confirm that it
>> supports TRIM
>>
>> The filesystem is classic UFS2.
>>
>> Is TRIM supported throught gmirror, or not?
>
> I think 8.4 is way too old for this. I have a number of 9.2 boxes - and
> even on those, TRIM doesn't propagate through gmirror *yet* (at the
> time, which is a few months ago I seem to remember reading 'It should
> work soon' - but 9.2 doesn't appear to).
>
> You can use gstat to show if any calls to BIO_DELETE are being made - i.e.
>
> gstat -d
>
> On the systems here the 'd/s' (which afaik is 'BIO_DELETE/second')
> remains stubbornly at zero :(

Thank you for pointing me to gstat -d. d/s is really zero all the time.

> The flipside is with decent SSD's these days TRIM isn't quite so
> important... Some may even argue that *not* issuing heaps of BIO_DELETE
> on the I/O channel is actually faster - so long as the SSD has time to
> play catchup and isn't pegged all the time, I think they may be right :-)

The SSDs in question are holding /var/db/mysql with heavily used InnoDB 
tables (grow only ibdata file, few deletes of MyISAM tables) so maybe 
there are not much delete commands at all.

Thank you.

Miroslav Lachman