>> - if a non-RAID SAS card is used, does it matter which card is chosen? >> Does btrfs work equally well with all of them? > > If you're using btrfs RAID, you need a HBA, not a RAID card. If the RAID > card can work as a HBA (usually labelled as JBOD mode) then you're good to > go. > > For example, HP CCISS controllers can't work in JBOD mode.
Would you know if they implement their own checksumming, similar to what btrfs does? Or if someone uses SmartArray (CCISS) RAID1, then they simply don't get the full benefit of checksumming under any possible configuration? I've had a quick look at what is on the market, here are some observations: - in many cases, IOPS (critical for SSDs) vary wildly: e.g. - SATA-3 SSDs advertise up to 85k IOPS, so RAID1 needs 170k IOPS - HP's standard HBAs don't support high IOPS - HP Gen8 SmartArray (e.g. P420) claims up to 200k IOPS - previous HP arrays (e.g. P212) support only 60k IOPS - many vendors don't advertise the IOPS prominently - I had to Google the HP site to find those figures quoted in some PDFs, they don't quote them in the quickspecs or product summary tables - Adaptec now offers an SSD caching function in hardware, supposedly drop it in the machine and all disks respond faster - how would this interact with btrfs checksumming? E.g. I'm guessing it would be necessary to ensure that data from both spindles is not cached on the same SSD? - I started thinking about the possibility that data is degraded on the mechanical disk but btrfs gets a good checksum read from the SSD and remains blissfully unaware that the real disk is failing, then the other disk goes completely offline one day, for whatever reason the data is not in the SSD cache and the sector can't be read reliably from the remaining physical disk - should such caching just be avoided or can it be managed from btrfs itself in a manner that is foolproof? How about the combination of btrfs/root/boot filesystems and grub? Can they all play nicely together? This seems to be one compelling factor with hardware RAID, the cards have a BIOS that can boot from any drive even if the other is offline. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
