From: NeilBrown <neilb@suse.com>
To: Avi Kivity <avi@scylladb.com>, linux-raid@vger.kernel.org
Subject: Re: raid0 vs. mkfs
Date: Mon, 28 Nov 2016 16:09:46 +1100 [thread overview]
Message-ID: <87oa108a1x.fsf@notabene.neil.brown.name> (raw)
In-Reply-To: <56c83c4e-d451-07e5-88e2-40b085d8681c@scylladb.com>
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On Mon, Nov 28 2016, Avi Kivity wrote:
> mkfs /dev/md0 can take a very long time, if /dev/md0 is a very large
> disk that supports TRIM/DISCARD (erase whichever is inappropriate).
> That is because mkfs issues a TRIM/DISCARD (erase whichever is
> inappropriate) for the entire partition. As far as I can tell, md
> converts the large TRIM/DISCARD (erase whichever is inappropriate) into
> a large number of TRIM/DISCARD (erase whichever is inappropriate)
> requests, one per chunk-size worth of disk, and issues them to the RAID
> components individually.
>
>
> It seems to me that md can convert the large TRIM/DISCARD (erase
> whichever is inappropriate) request it gets into one TRIM/DISCARD (erase
> whichever is inappropriate) per RAID component, converting an O(disk
> size / chunk size) operation into an O(number of RAID components)
> operation, which is much faster.
>
>
> I observed this with mkfs.xfs on a RAID0 of four 3TB NVMe devices, with
> the operation taking about a quarter of an hour, continuously pushing
> half-megabyte TRIM/DISCARD (erase whichever is inappropriate) requests
> to the disk. Linux 4.1.12.
Surely it is the task of the underlying driver, or the queuing
infrastructure, to merge small requests into large requests.
Why should this have anything do to with RAID0?
NeilBrown
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next prev parent reply other threads:[~2016-11-28 5:09 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-11-27 15:24 raid0 vs. mkfs Avi Kivity
2016-11-27 17:09 ` Coly Li
2016-11-27 17:25 ` Avi Kivity
2016-11-27 19:25 ` Doug Dumitru
2016-11-28 4:11 ` Chris Murphy
2016-11-28 7:28 ` Avi Kivity
2016-11-28 7:33 ` Avi Kivity
2016-11-28 5:09 ` NeilBrown [this message]
2016-11-28 6:08 ` Shaohua Li
2016-11-28 7:38 ` Avi Kivity
2016-11-28 8:40 ` NeilBrown
2016-11-28 8:58 ` Avi Kivity
2016-11-28 9:00 ` Christoph Hellwig
2016-11-28 9:11 ` Avi Kivity
2016-11-28 9:15 ` Coly Li
2016-11-28 17:47 ` Shaohua Li
2016-11-29 21:14 ` NeilBrown
2016-11-29 22:45 ` Avi Kivity
2016-12-07 5:08 ` Mike Snitzer
2016-12-07 11:50 ` Coly Li
2016-12-07 12:03 ` Coly Li
2016-12-07 16:59 ` Shaohua Li
2016-12-08 16:44 ` Coly Li
2016-12-08 19:19 ` Shaohua Li
2016-12-09 7:34 ` Coly Li
2016-12-12 3:17 ` NeilBrown
2017-06-29 15:15 ` Avi Kivity
2017-06-29 15:31 ` Coly Li
2017-06-29 15:36 ` Avi Kivity
2017-01-22 18:01 ` Avi Kivity
2017-01-23 12:26 ` Coly Li
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