linux-ext4 - Re: Ext4: batched discard support

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Message-ID: <AANLkTi=E8dT8+cSge=uPGP4ahrJn4GkZnSxJq45JQcHA@mail.gmail.com>
Date:	Fri, 23 Jul 2010 11:30:30 -0400
From:	Greg Freemyer <greg.freemyer@...il.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	"Ted Ts'o" <tytso@....edu>, Lukas Czerner <lczerner@...hat.com>,
	eshishki@...hat.com, rwheeler@...hat.com,
	linux-ext4@...r.kernel.org, sandeen@...hat.com
Subject: Re: Ext4: batched discard support - simplified version

On Fri, Jul 23, 2010 at 11:13 AM, Jeff Moyer <jmoyer@...hat.com> wrote:
> "Ted Ts'o" <tytso@....edu> writes:
>
>> On Wed, Jul 07, 2010 at 09:53:30AM +0200, Lukas Czerner wrote:
>>>
>>> Hi all,
>>>
>>> since my last post I have done some more testing with various SSD's and the
>>> trend is clear. Trim performance is getting better and the performance loss
>>> without trim is getting lower. So I have decided to abandon the initial idea
>>> to track free blocks within some internal data structure - it takes time and
>>> memory.
>>
>> Do you have some numbers about how bad trim actually might be on
>> various devices?
>
> I'll let Lukas answer that when he gets back to the office next week.
> The performance of the trim command itself varies by vendor, of course.
>
>> I can imagine some devices where it might be better (for wear
>> levelling and better write endurance if nothing else) where it's
>> better to do the trim right away instead of batching things.
>
> I don't think so.  In all of the configurations tested, I'm pretty sure
> we saw a performance hit from doing the TRIMs right away.  The queue
> flush really hurts.  Of course, I have no idea what you had in mind for
> the amount of time in between batched discards.

It was my understanding that way back when, Intel was pushing for the
TRIMs to be right away.  That may be why they never fully implemented
the TRIM command to accept more than one sectors worth of vectorized
data.  (That is still multiple ranges per discard, just not
hundreds/thousands.)

Along those lines, does this patch create multi-sector discard trim
commands when there is a large list of discard ranges (ie. thousands
of ranges to discard)?  And if so, does it have a blacklist for SSDs
like the Intel that don't implement the multi-sector payload part of
the spec?

Greg
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