Date: Sun, 16 Dec 2007 20:40:18 +1100 From: Darren Reed <darrenr@freebsd.org> To: ticso@cicely.de Cc: freebsd-current@freebsd.org, Ivan Voras <ivoras@freebsd.org> Subject: Re: ZFS melting under postgres... Message-ID: <4764F282.7030706@freebsd.org> In-Reply-To: <20071216024259.GI48684@cicely12.cicely.de> 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>
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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. Darren
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