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. ScottReceived on Sun Dec 16 2007 - 16:18:12 UTC
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