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.
On top of that bcache device, I decided to add an LVM layer (making /dev/bcache* a PV), which is really helpful i.e. with the base problem of this thread: You can create additional bcache devices and pvmove your data to the new PV, without interrupting production.
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.
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