Date: Sun, 19 Nov 2017 11:32:41 +0100 From: Andre Albsmeier <andre@fbsd.e4m.org> To: freebsd-hackers@freebsd.org Cc: andre@fbsd.e4m.org Subject: gmirror synchronising is very slow due to frequent metadata updates Message-ID: <20171119103241.GA20588@voyager>
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I am running a 2 TB gmirror with two disks. I replaced one disk and the
mirror started to synchronise. I noticed that the sync rate was about
50 MB/s which is a lot lower than the more than 120 MB/s which are
possible with this hardware.
The reason for this are the very frequently happening metadata updates
done in sys/geom/mirror/g_mirror.c:
if (sync->ds_offset_done + (MAXPHYS * 100) < offset) {
/* Update offset_done on every 100 blocks. */
sync->ds_offset_done = offset;
g_mirror_update_metadata(disk);
With MAXPHYS being 128kB this would mean that metadata is updated every
12 MB -- or 4 times a second @ 50 MB/s.
Out of curiosity I changed this to MAXPHYS * 2000 which means that the
updating is done every 240 MB. Now the disks are synchronising with
approx. 105 MB/s...
Is there any reason why these metadata updates are done so often and
what are the risks having them more infrequently (as I do now)?
-Andre
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