Re: setting up raid10 with more than 4 drives
От | PFC |
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Тема | Re: setting up raid10 with more than 4 drives |
Дата | |
Msg-id | op.ts5b3kikcigqcu@apollo13 обсуждение исходный текст |
Ответ на | Re: setting up raid10 with more than 4 drives ("Luke Lonergan" <LLonergan@greenplum.com>) |
Ответы |
Autodetect of software RAID1+0 fails
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Список | pgsql-performance |
On Wed, 30 May 2007 16:36:48 +0200, Luke Lonergan <LLonergan@greenplum.com> wrote: >> I don't see how that's better at all; in fact, it reduces to >> exactly the same problem: given two pieces of data which >> disagree, which is right? > > The one that matches the checksum. - postgres tells OS "write this block" - OS sends block to drives A and B - drive A happens to be lucky and seeks faster, writes data - student intern carrying pizzas for senior IT staff trips over power cord* - boom - drive B still has old block Both blocks have correct checksum, so only a version counter/timestamp could tell. Fortunately if fsync() is honored correctly (did you check ?) postgres will zap such errors in recovery. Smart RAID1 or 0+1 controllers (including software RAID) will distribute random reads to both disks (but not writes obviously). * = this happened at my old job, yes they had a very frightening server room, or more precisely "cave" ; I never went there, I didn't want to be the one fired for tripping over the wire... From Linux Software RAID howto : - benchmarking (quite brief !) http://unthought.net/Software-RAID.HOWTO/Software-RAID.HOWTO-9.html#ss9.5 - read "Data Scrubbing" here : http://gentoo-wiki.com/HOWTO_Install_on_Software_RAID - yeah but does it work ? (scary) http://bugs.donarmstrong.com/cgi-bin/bugreport.cgi?bug=405919 md/sync_action This can be used to monitor and control the resync/recovery process of MD. In particular, writing "check" here will cause the array to read all data block and check that they are consistent (e.g. parity is correct, or all mirror replicas are the same). Any discrepancies found are NOT corrected. A count of problems found will be stored in md/mismatch_count. Alternately, "repair" can be written which will cause the same check to be performed, but any errors will be corrected.
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