> On 19/12/2009 01:05, Bernd Schubert wrote: > > On Saturday 19 December 2009, Matt Tehonica wrote: > >> I have a 4 disk RAID5 using a 2048K chunk size and using XFS > > > > 4 disks is a bad idea. You should have 2^n data disks, but you have 2^1 > + 1 = > > 3 data disks. As parity information are calculated in the power of two > and > > blocks are written in the power of two > > Sorry, but where did you get that from? p = d1 xor d2 xor d3 has nothing > to do with powers of two, and I'm sure blocks are written whenever they > need to be, not in powers of two. Yeah, I was scratching my head over that one, too. It sounded bogus to me, but I didn't want to open my mouth, so to speak, when I was unsure of myself. Being far from expert in the matter, I can't be certain, but I surely can think of no reason why writes would occur in powers of two, or even be more efficient because of it. > > you probably have read operations, > > when you only want to write. > > That will depend on how much data you're trying to write. With 3 data > discs and a 2M chunk size, writes in multiples of 6M won't need reads. > Writing a 25M file would therefore write 4 stripes and need to read to > do the last 1M. With 4 data discs, it'd be 8M multiples, and you'd write > 3 stripes and need a read to do the last 1M. No difference. I hadn't really considered this before, and I am curious. Of course there is no reason for md to read a stripe marked as being in use if the data to be written will fill an entire stripe. However, does it only apply this logic if the data will completely fill a stripe? The most efficient use of disk space of course will be accomplished if the system reads the potential partially used target stripe whenever the write buffer contains even 1 chunk less than a full stripe, but the most efficient write speeds will only check on writing to a partially used stripe if the write buffer contains less than half a stripe worth of data. Does anyone know which is the case? -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
