Re: [xfs_check Out of memory: ]

From: Matthias Schniedermeyer <ms@citd.de>
To: Stan Hoeppner <stan@hardwarefreak.com>
Cc: Stor?? <289471341@qq.com>, Jeff Liu <jeff.liu@oracle.com>,
	xfs@oss.sgi.com
Subject: Re: [xfs_check Out of memory: ]
Date: Mon, 30 Dec 2013 12:27:43 +0100	[thread overview]
Message-ID: <20131230112743.GA16889@citd.de> (raw)
In-Reply-To: <52C0D281.7040704@hardwarefreak.com>

On 29.12.2013 19:55, Stan Hoeppner wrote:
> On 12/29/2013 3:50 AM, Dave Chinner wrote:
> ...
> > I think you are forgetting that developer time is *expensive* and
> > *scarce*. This is essentially a solved problem: An SSD in a USB3
> > enclosure as a temporary swap device is by far the most cost
> > effective way to make repair scale to arbitrary amounts of metadata.
> > It certainly scales far better than developer time and testing
> > resources...
> 
> Now this is an interesting idea Dave.  I hadn't considered temporary
> swap.  Would USB be reliable enough for this?  I've seen lots problem
> reports with folks using USB storage with Linux, random disconnections
> and what not.

It's certainly a problem with several variables.
- Quality of USB-Stack (should be quite good nowadays, but there can 
always be (new) bugs)
- Quality of SATA <-> USB(3) Chip
- Quality of SSD itself

And with Quality i mean everything from physical chip up to firmware.

I'm not quite sure what happens when SWAP crapps out. I think it can be 
everything from "machine goes dead" down to "all programs with pages in 
swap are terminated".

I've transfert quite a few TB over USB3 and i can say, it mostly works. 
But random disconnects happen and you can't really be sure which part is 
the problem as it only happens rarely.

For e.g. currently i have a HDD that randomly but seldomly craps out in 
an USB3 enclosure, after coping a few hundert GB. The drive works 
flawlessly when connected directly by SATA to a (different) computer, at 
least i haven't had a failure after i moved the drive. Is it the drive, 
chip in enclose, firmware between HDD & enclosure not playing nice (like 
too high command timeouts on HDD side and too low on enclosure-sde), 
USB3 stack. Can't really tell.

So:
I would consides SWAP on a device connected via USB3 to be on the risky 
side.

I would validate beforehand if that specific combination of 
xhci/enclosure chip/SSD survies a prolonged time of "high I/O stress".
Like several days of full bandwidth/random I/O.

-- 

Matthias

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