Les Mikesell wrote: > Evren Yurtesen wrote: > >>> Raid5 doesn't distribute disk activity - it puts the drives in >>> lockstep and is slower than a single drive, especially on small writes >>> where it has to do extra reads to re-compute parity on the existing data. >>> >> I am confused, when a write is done the data is distributed in the disks >> depending on the stripe size you are using. When you start reading the >> file, you are reading from 5 different disks. So you get way better >> performance for sure on reads. >> > > The stripe effect only comes into play on files large enough to span > them and not at all for directory/inode accesses which is most of what > you are doing. Meanwhile you have another head tied up checking the > parity and for writes of less than a block you have to read the existing > contents before the write to re-compute the parity. > > Actually most (all?) raid 5 systems I've met don't check parity on read - they rely on the drive indicating a failed read. However the write splice penalty for raid 5 can still be pretty high (seek, read, insert new data, compute new parity, write data, seek, write parity) - and that's on top of the fact that the OS did it's own logical read, check permissions, insert, write to update directories and the like.
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