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Re: [xfs_check Out of memory: ]

To: Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx>
Subject: Re: [xfs_check Out of memory: ]
From: Matthias Schniedermeyer <ms@xxxxxxx>
Date: Mon, 30 Dec 2013 12:27:43 +0100
Cc: Dave Chinner <david@xxxxxxxxxxxxx>, Arkadiusz Mi??kiewicz <arekm@xxxxxxxx>, Stor?? <289471341@xxxxxx>, Jeff Liu <jeff.liu@xxxxxxxxxx>, xfs@xxxxxxxxxxx
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In-reply-to: <52C0D281.7040704@xxxxxxxxxxxxxxxxx>
References: <tencent_3F12563342ED1D4E049D1123@xxxxxx> <201312270907.22638.arekm@xxxxxxxx> <20131227224212.GK20579@dastard> <201312280020.39244.arekm@xxxxxxxx> <20131229095033.GL20579@dastard> <52C0D281.7040704@xxxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
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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