Date: Sun, 16 Dec 2007 10:07:23 -0700 From: Scott Long <scottl@samsco.org> To: darrenr@freebsd.org Cc: freebsd-current@freebsd.org, ticso@cicely.de, Ivan Voras <ivoras@freebsd.org> Subject: Re: ZFS melting under postgres... Message-ID: <47655B4B.6010902@samsco.org> In-Reply-To: <4764F282.7030706@freebsd.org> References: <06CAC7FC-DB58-441D-A6E0-76D1D8133393@tamu.edu> <86ir31xwlu.fsf@ds4.des.no> <ADCCD5E6-A792-49B9-A346-753176C12F2E@tamu.edu> <fjuljp$cvb$1@ger.gmane.org> <476343B4.8080208@FreeBSD.org> <fk09p8$b16$1@ger.gmane.org> <86tzmk54tt.fsf@ds4.des.no> <fk0ue7$bp$1@ger.gmane.org> <476419CD.9070401@terranova.net> <fk1j0l$o4l$1@ger.gmane.org> <20071216024259.GI48684@cicely12.cicely.de> <4764F282.7030706@freebsd.org>
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Darren Reed wrote: > Bernd Walter wrote: > ... >> One problem is with the data blocks beeing that big, when writing >> 512 Byte you effectifly do a read-modify-write of a larger physical >> block. >> This can be handled quite well with larger FS block. >> The much bigger problem is with power loss when writing such a >> maintenence block. >> You loose a very large area of logical blocks when this fails, >> since a 4k maintenence block contains the allocation for several hundert >> kB of logical data blocks. >> In other words - you possibly loose data blocks that were not written >> a long time and the database wouldn't expect a problem with that data. >> Even for ZIL it is very questionable if you loose a large data area, >> since the purpose is to have the data that was already sinced readable >> after a power loss. > ... > > ZFS doesn't suffer from this problem because the design > is to always write a new section of data rather than > over write "current" data. > > So if you lose power in the middle of a write to a data > block, there is no damage to the old data. ... except with disks that write sectors via read-update-write on whole tracks at a time (i.e. all SATA/ATA disks and probably more and more SAS/SCSI disks as well these days). The speed and density optimizations that have been introduced to disks in the past 10 years don't come for free; they directly impact reliability. That's why you don't ever, ever want to loose power to a disk subsystem that you consider critical. Scott
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