On 27 Jun 2017, Jens-U. Mozdzen verbalised: > Hi *, > > Zitat von Henk Slager <eye1tm@xxxxxxxxx>: >> [...] There >> is someone on the list who uses bcache on top of MD RAID5 AFAIR. > > that would probably be me: backing device was an MD-RAID6 (we've > switched to RAID1 in the meantime because the data fits on a single > disk (netto) and we needed the HDD slots) and caching device is an > MD-RAID1 consisting of two SSDs. Me too, but the fact that my bcache cache device exploded with "missing magic" errors after less than a week makes me perhaps not a very good advert for this. >> I think I would choose to add bcache to each of the four harddisks, > > If you'd do that with a single caching device, you're in for contention. My gut feeling tells me that running a single bcache > backing device/caching device combo on top of MD-RAID is less straining than running MD-RAID across a bunch of bcache devices with > a common caching device: The MD advantage of spreading the load across multiple "disks" is countered by accessing a common SSD. It all depends what your speed is like: if the load is at all seeky, an SSD's zero seek time will dominate. md atop bcache will certainly have far more SSD-write overhead than bcache atop md, because every time md does a full-stripe read or read-modify-write cycle you'll be burning holes in the SSD for no real speed benefit whatsoever... One thing to note about working atop md is that you do have to be careful about alignment: you don't want every md write to incur a read-modify-write cycle after you've gone to some lengths to tell your filesystems what the RAID offset is. You should compute an appropriate --data-offset for not only the array configuration now (so a multiple of your chunk size, at least, and possibly of your stripe size) but if you are planning to add more data spindles you should figure out a (larger) --data-offset that will retain appropriate alignment for plausible larger sets of spindles you might grow to in the future. (Also note that bcache does *not* communicate the RAID geometry through to overlying filesystems -- if you want decent write performance on RAID-5 or -6 you'll need to do that yourself, via mke2fs -E stripe_width/stride, mkfs.xfs sunit and swidth arguments, cryptsetup --align-payload, etc. You can use btrace on the underlying real block device to see if the requests look aligned after you're done, but before populating it with data. :) ) -- NULL && (void) -- To unsubscribe from this list: send the line "unsubscribe linux-bcache" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
