linux-kernel - Re: [PATCH 1/2] fs: Do not dispatch FITRIM through separate super

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Date:	Wed, 24 Nov 2010 23:41:26 -0500
From:	Greg Freemyer <greg.freemyer@...il.com>
To:	Mark Lord <kernel@...savvy.com>
Cc:	"Ted Ts'o" <tytso@....edu>, Lukas Czerner <lczerner@...hat.com>,
	Steven Whitehouse <swhiteho@...hat.com>,
	James Bottomley <James.Bottomley@...e.de>,
	Christoph Hellwig <hch@...radead.org>,
	Matthew Wilcox <matthew@....cx>,
	Josef Bacik <josef@...hat.com>, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	sandeen@...hat.com
Subject: Re: [PATCH 1/2] fs: Do not dispatch FITRIM through separate super_operation

On Wed, Nov 24, 2010 at 9:48 PM, Mark Lord <kernel@...savvy.com> wrote:
> On 10-11-19 11:30 AM, Ted Ts'o wrote:
>>
>> On Fri, Nov 19, 2010 at 04:44:33PM +0100, Lukas Czerner wrote:
>>>>
>>>> But, oddly, it _is_ the default for mke2fs -t ext4,
>>>> which really threw me for a loop recently.
>>>>
>>>> I though my system had locked up when suddenly everything
>>>> went dead for a very long time (many minutes) while installing a
>>>> new system.
>>
>> Yeah, the assumption was doing a single big discard (which is all
>> mke2fs is doing) should be fast.  At least on sanely implemented SSD's
>> (i.e., like the Intel X25-M) it should be, since all that should
>> require is a flash write to the global mapping table, declaring all of
>> the blocks as free.
>
> But mke2fs probably is NOT doing a "single big discard", because for SATA
> the
> TRIM command is limited to 64K sectors per range.. and the in-kernel TRIM
> code only ever does single ranges..
>
> So doing a discard over an entire drive-encompassing partition, say.. 100GB,
> will require 3000+ individual TRIM commands.  At (say) 200msecs each, that
> adds up to about ten minutes of execution time.  Or less if the drive is
> faster than that.
>
> Whereas.. grouping them into 64-ranges per trim, could reduce the execution
> time down to perhaps 1/50th of that, or in the range of 10-20 seconds
> instead.
>
> Cheers

Mark,

With recent kernels, this is supposed to work as you describe.  ie. 64
contiguous ranges per trim command.

If you see a significant speed difference between mke2fs and running
wiper.sh on that same filesystem immediately after formatting, then
their is likely a bug worth chasing.

Are you seeing an actual speed difference, or just assuming there is
one?  If mke2fs is slower than wiper.sh, what kernel are you testing
with?

Greg
-- 
Greg Freemyer
Head of EDD Tape Extraction and Processing team
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