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Re: [Qemu-block] [PATCH] block: don't probe zeroes in bs->file by defaul

From: Eric Blake
Subject: Re: [Qemu-block] [PATCH] block: don't probe zeroes in bs->file by default on block_status
Date: Fri, 11 Jan 2019 10:05:17 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.3.1

On 1/11/19 10:02 AM, Eric Blake wrote:

> Even a dumb most-recent use cache will speed this up: both the second
> and third queries above can be avoided because we know that both 0x40000
> and 0x30000 the second query at 0x40000 can be skipped (0x40000 is
> between our most recent lseek at 0x20000 and hole at 0x10000)
> Make the cache slightly larger, or use a bitmap with 2 bits per cluster
> (tracking unknown, known-data, known-hole), with proper flushing of the
> cache as we write to the image, or whatever, and we should automatically
> get some performance improvements by using fewer lseek() anywhere that
> we remember what previous lseek() already told us, with no knobs needed.

In fact, block/iscsi.c has such a cache; look at iscsi_allocmap_update()
and friends; maybe we should generalize that code in order to reuse the
same concepts in file-posix.c.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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