On Tue, Feb 24, 2015 at 12:29 AM, Markus Stockhausen <stockhausen@xxxxxxxxxxx> wrote: >> Von: linux-raid-owner@xxxxxxxxxxxxxxx [linux-raid-owner@xxxxxxxxxxxxxxx]" im Auftrag von "Roman Mamedov [rm@xxxxxxxxxxx] >> Gesendet: Dienstag, 24. Februar 2015 00:58 >> An: linux-raid@xxxxxxxxxxxxxxx >> Betreff: RAID6 write I/O amplification? >> >> Hello, >> >> Got a bit of a "how does it actually work" question... >> >> Suppose I have an MD RAID6 of 8 drives, with 64KB chunk size. >> >> I am rewriting a 4KB filesystem sector somewhere on that RAID (not crossing >> the stripe boundary). >> >> What's the amount of disk I/O in total this will result in? >> >> I assume the RAID will need to read data from all drives, recompute parity, >> then write to the data stripe where the updated piece happened to be, and also >> write to two parity stripes. >> >> Is this done at a stripe granularity, so 6x64KB reads, 3x64KB writes? >> Or down to individual sectors (pages), i.e. 6x4KB reads, 3x4KB writes? >> Or am I describing this algorithm correctly at all? > > Implementation will work on "internal" stripe granularity and that is 4K > So your case will be 6x4KB read + 3x4KB write. Having said that, does it mean that following description of "chunk size" is wrong: '[chunk size] is the smallest "atomic" mass of data that can be written to the devices' since in this case chunk size is 64KB but 4KB is written atomically (?). I have find it in the kernel.org wiki page [1] --- Alireza 1. https://raid.wiki.kernel.org/index.php/RAID_setup -- 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
