[CentOS] cyrus spool on btrfs?

[John R Pierce]

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.

...

> 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