Stephan van Hienen wrote: > > Options are: left-asymmetric, left-symmetric, right-asymmetric, > right-symmetric, la, ra, ls, rs. > The default is left-symmetric. > > What is the difference between this ? > and why do i want to use anything else than the default ? Hi Stephan, I'm forwarding a message that I posted to linux-kernel a little while back...someone else had asked some very similar questions...I hope this helps... -- Paul ---------- Forwarded message ---------- Date: Thu, 16 Jan 2003 14:47:16 -0500 (EST) From: Paul Clements <kernel@steeleye.com> Reply-To: Paul.Clements@steeleye.com To: Ezra Nugroho <ezran@goshen.edu> Cc: linux-kernel@vger.kernel.org Subject: Re: raid 5 algorithm docs On 16 Jan 2003, Ezra Nugroho wrote: > I don't know if linux-raid mailing list is still active, the archive > sites look very old... Yes, linux-raid is still alive and kicking... it's at: linux-raid@vger.kernel.org > Raidtab's man page tells about left/right (a)synchronous algorithm, but > doesn't tell how they differ. I haven't seen much documentation either, but here's what I got from looking at the code (BTW, the relevant section of code is raid5.c:raid5_compute_sector(), if you're curious): Basically, the left/right refers to how the parity information is laid out and the symmetric/asymmetric refers to how the data is laid out. The "left" algorithms start with parity on the last disk, moving the parity one disk closer to the first disk for each stripe (wrapping as necessary). The "right" algorithms do just the opposite, starting with parity on the first disk, moving it one disk closer to the last disk for each stripe (wrapping as necessary). The "asymmetric" algorithms place the data blocks for a given stripe in simple sequential order, skipping over the parity block as necessary and always starting with the first data block of a stripe on the first disk. The "symmetric" algorithms differ from this in that they do not always place the first block of a stripe on the first disk, but continue to lay out the data blocks in sequential disk order, simply wrapping back to the first disk when it's necessary. So the symmetric algorithms tend to give better performance on large sequential reads, for example, since the actual disk reads are evenly spread across all disks. There's a pretty good diagram of the left-symmetric disk layout here: www.pdl.cmu.edu/PDL-FTP/Declustering/ASPLOS.ps (see figure 2-1) -- Paul Clements Paul.Clements@SteelEye.com - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
