linux-kernel - Re: [PATCH 1/4] block: implement compatible DISCARD support

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Date:	Thu, 11 Feb 2010 09:06:48 -0500
From:	Christoph Hellwig <hch@...radead.org>
To:	Dmitry Monakhov <dmonakhov@...nvz.org>
Cc:	Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org, jens.axboe@...cle.com
Subject: Re: [PATCH 1/4] block: implement compatible DISCARD support

On Thu, Feb 11, 2010 at 04:45:13PM +0300, Dmitry Monakhov wrote:
> >> The question is should it be sync or async it is not easy to design
> >> simple async interface so let's use sync by default
> >> BTW: That's why blkdev_issue_barrier has to wait by default. 
> In fact wait is the only interface for issue_barrier.
> > This is going to kill performance.
> But it may be reasonable to allow caller to choose would it
> wait and work fair, or to cheat in a name of performance.

It's not a cheat.  The discard is a _hint_ to the hardware that it
can reclaim space.  Think become a bit different when we start relying
on the zeroing behaviour for hardware that supports it, but so far
we don't.  And given that out of two TRIM capable devices I have one
does not reliably zero the trimmed regions I'm not sure it's a good
idea to rely on that yet, either.

> libata-scsi.c: ata_scsi_write_same_xlat
>                   ata_set_lba_range_entries
> It's was not obvious from the first glance. But it is the way how it
> works for now. But seems what we still optimize things a bit
> 1) alloc page with GFP_HIGHUSER (because x86 arch still used)

I'm not sure it's worth over-optimization this.

> 2) Share page between eight bios.

If we introduce the common completion handler for a batch of bios as
your patch does we can do that.

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