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

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Date:	Thu, 18 Nov 2010 19:34:30 -0500
From:	Mark Lord <kernel@...savvy.com>
To:	"Martin K. Petersen" <martin.petersen@...cle.com>
CC:	Greg Freemyer <greg.freemyer@...il.com>,
	James Bottomley <James.Bottomley@...e.de>,
	Jeff Moyer <jmoyer@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Matthew Wilcox <matthew@....cx>,
	Josef Bacik <josef@...hat.com>,
	Lukas Czerner <lczerner@...hat.com>, tytso@....edu,
	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 10-11-18 06:52 PM, Martin K. Petersen wrote:
>>>>>> "Mark" == Mark Lord<kernel@...savvy.com>  writes:
> Mark>  If FITRIM is still issuing single-range-at-a-time TRIMs, then I'd
> Mark>  call that a BUG that needs fixing.  Doing TRIM like that causes
> Mark>  tons of unnecessary ERASE cycles, shortening the SSD lifetime.  It
> Mark>  really needs to batch them into groups of (up to) 64 ranges at a
> Mark>  time (64 ranges fits into a single 512-byte parameter block).
>
> We don't support coalescing discontiguous requests into one command. But
> we will issue contiguous TRIM requests as big as the payload can
> handle. That's just short of two gigs per command given a 512-byte
> block.
>
> I spent quite a bit of time trying to make coalescing work in the
> spring. It got very big and unwieldy. When we discussed it at the
> filesystem summit the consensus was that it was too intrusive to the I/O
> stack, elevators, etc.

Surely if a userspace tool and shell-script can accomplish this,
totally lacking real filesystem knowledge, then we should be able
to approximate it in kernel space?

This is FITRIM we're talking about, not the on-the-fly automatic TRIM.

FITRIM could perhaps use a similar approach to what wiper.sh does:
reserve a large number of free blocks, and issue coalesced TRIM(s) on them.

The difference being, it could walk through the filesystem,
trimming in sections, rather than trying to reserve/trim the entire
freespace all in one go.

Over-thinking it???
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