Re: 5.1.21 Dell 2950 terrible swraid5 I/O performance with swraid on top of Perc 5/i raid0/jbod

[Roman Mamedov <rm@romanrm.net>]

On Mon, 19 Aug 2019 00:08:23 -0700
Marc MERLIN <marc@merlins.org> wrote:

> > Default Cache Policy: WriteBack, ReadAheadNone, Direct, No Write Cache if Bad BBU
> > Current Cache Policy: WriteBack, ReadAheadNone, Direct, No Write Cache if Bad BBU
> > Default Access Policy: Read/Write
> > Current Access Policy: Read/Write
> > Disk Cache Policy   : Disabled

So does it have a BBU? (Battery backup unit)

> I tried to disable the card's write cache to let linux and its 32GB of
> RAM, do it better, but I didn't see a real improvement:

I'd expect that on the contrary, you should look for ways to enable it, and
force-enable even without that BBU (in case of lack of one), because it feels
like what you did is disable disks' own write buffering, and not (only) the
card's!

What you are observing seems to me like what "dd" does with "oflag=dsync" (and
comparable performance that it gets). Definitely feels like it's in some
"extra safe mode" and, say, every 4KB piece of data leads to full flush to
disk before accepting to write the next 4KB.

More things to try, check if it's possible to set up disks not as 1-member
RAID0, but 1-member "linear" ("JBOD"), or even 1-member RAID1, who knows maybe
some of this would work better.

-- 
With respect,
Roman