linux-ext4 - Re: [RFC] Ext3 online defrag

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Date:	Fri, 27 Oct 2006 18:05:57 +0400
From:	Alex Tomas <alex@...sterfs.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Alex Tomas <alex@...sterfs.com>, sho@...s.nec.co.jp, tytso@....edu,
	jack@...e.cz, linux-fsdevel@...r.kernel.org,
	linux-ext4@...r.kernel.org
Subject: Re: [RFC] Ext3 online defrag

>>>>> Eric Sandeen (ES) writes:

 ES> Alex Tomas wrote:
 >> 3) scalable reservation
 >> required for delayed allocation to avoid -ENOSPC at flush time.
 >> current version uses per-sb spinlock.

 ES> Can you elaborate on this issue?  Shouldn't delayed allocation
 ES> decrement free space immediately, and only the actual block location
 ES> choice is delayed?  Or is this due to potential extra metadata space
 ES> required as blocks are allocated?

exactly. in this case, reservation has nothing to do with allocation
or preallocation of real blocks. this is just a *per-sb counter* of
blocks reserved for allocation at flush time. it includes all
non-allocated-yet blocks and metadata needed to allocate them (bitmaps,
group descriptors, blocks extent tree, etc). the previous version
of mballoc has reservation, but it doesn't scale very well being
a single global counter protected by the spinlock. at least, in many
regular loads I observed the reservation function in top30 of oprofile.

thanks, Alex
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