On 9/8/2017 2:36 PM, Valeri Galtsev wrote:
With all due respect, John, this is the same as hard drive cache is not backed up power wise for a case of power loss. And hard drives all lie about write operation completed before data actually are on the platters. So we can claim the same: hard drives are not suitable for RAID. I implied to find out from experts in what respect they claim SSDs are unsuitable for hardware RAID as opposed to mechanical hard drives. Am I missing something?
major difference is, SSD's do a LOT more write buffering as their internal write blocks are on the order of a few 100KB, also they extensively reorder data on the media, both for wear leveling and to minimize physical block writes so there's really no way the host and/or controller can track whats going on.
enterprise hard disks do NOT do hidden write buffering, its all fully managable via SAS or SATA commands. desktop drives tend to lie about it to achieve better performance. I do NOT use desktop drives in raids.
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And one may want to adjust stripe size to be resembling SSDs internals, as default is for hard drives, right?
as the SSD physical data blocks have no visible relation to logical block numbers or CHS, its not practical to do this. I'd use a fairly large stripe size, like 1MB, so more data can be sequentially written to the same device (even tho the device will scramble it all over as it sees fit).
-- john r pierce, recycling bits in santa cruz _______________________________________________ CentOS mailing list CentOS@xxxxxxxxxx https://lists.centos.org/mailman/listinfo/centos
