3 ms·

Wouldn't you get higher read throughput (2x for R1, less for R5) by reading the parity disks as well?

dspillett 13y ago
Not for R5, because in a non-degraded array the parity blocks do not contain anything you have not read from other blocks in that stripe and unlike the data blocks can not be used as meaningful data without at least one other block in the stripe being read. You do get the speed benefit fromt he striping of course: RAID5 over three drives performs similarly to RAID0 over two for read operations (though be warey of write performance if your I/O pattern is write heavy, particularly for many small randomly positioned writes, as RAID5 has issues here). For R1 you can in theory use an elevtor algorithm to chose which or the two mirrors serves a given read depending on how close the block may be to the most recently accessed blocks on each device, reducing average head movement distance. For bulk reads or an I/O pattern including much writing this will make little or no difference, but for significant random access read patterns you might see a measurable (perhaps even noticable to a human user) latency improvement. Last time I tested this myself I found that the Linux software RAID arrangement did not perform any such optimisation for RAID1. Results like https://raid.wiki.kernel.org/index.php/Performance https://raid.wiki.kernel.org/index.php/Performance agree with my findings, though they do suggest that with certain layout options the newer RAID10 single-level driver provides such optimisation for reads (at the expense of slower write performance). Linux's md-raid "raid10" module is not your traditional RAID10 (a nested arrangement of RAID 1 and RAID0, a stripset of mirrors needing at least four drives) - it is a single level driver that offers greater layout flexibility with regard to the copies of data on each device and even supports operation on three devices (this operates similarly to what IBM RAID controllers call RAID1E and some others call RAID0+1) or even just two devices (in which case it behaves as RAID1 but with the extra layout options). See http://neil.brown.name/blog/20040827225440 http://neil.brown.name/blog/20040827225440 amongst other references. IIRC you should avoid the "far" layour for your boot devices as boot loaders don't tend to understand it.

Not for R5, because in a non-degraded array the parity blocks do not contain anything you have not read from other blocks in that stripe and unlike the data blocks can not be used as meaningful data without at least one other block in the stripe being read. You do get the speed benefit fromt he striping of course: RAID5 over three drives performs similarly to RAID0 over two for read operations (though be warey of write performance if your I/O pattern is write heavy, particularly for many small randomly positioned writes, as RAID5 has issues here). For R1 you can in theory use an elevtor algorithm to chose which or the two mirrors serves a given read depending on how close the block may be to the most recently accessed blocks on each device, reducing average head movement distance. For bulk reads or an I/O pattern including much writing this will make little or no difference, but for significant random access read patterns you might see a measurable (perhaps even noticable to a human user) latency improvement. Last time I tested this myself I found that the Linux software RAID arrangement did not perform any such optimisation for RAID1. Results like https://raid.wiki.kernel.org/index.php/Performance https://raid.wiki.kernel.org/index.php/Performance agree with my findings, though they do suggest that with certain layout options the newer RAID10 single-level driver provides such optimisation for reads (at the expense of slower write performance). Linux's md-raid "raid10" module is not your traditional RAID10 (a nested arrangement of RAID 1 and RAID0, a stripset of mirrors needing at least four drives) - it is a single level driver that offers greater layout flexibility with regard to the copies of data on each device and even supports operation on three devices (this operates similarly to what IBM RAID controllers call RAID1E and some others call RAID0+1) or even just two devices (in which case it behaves as RAID1 but with the extra layout options). See http://neil.brown.name/blog/20040827225440 http://neil.brown.name/blog/20040827225440 amongst other references. IIRC you should avoid the "far" layour for your boot devices as boot loaders don't tend to understand it.