linux-kernel - Re: [ANNOUNCE] Ramback: faster than a speeding bullet

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Date:	Fri, 14 Mar 2008 11:47:04 -0400
From:	Ric Wheeler <ric@....com>
To:	Theodore Tso <tytso@....edu>,
	Benny Amorsen <benny+usenet@...rsen.dk>,
	linux-kernel@...r.kernel.org
Subject: Re: [ANNOUNCE] Ramback: faster than a speeding bullet

Theodore Tso wrote:
> On Fri, Mar 14, 2008 at 12:41:31PM +0100, Benny Amorsen wrote:
>> Ric Wheeler <ric@....com> writes:
>>
>>> The only really safe default is to disable the write cache by default
>>> or possibly dynamically disable the write cache when barriers are not
>>> supported by a drive. Both have a severe performance impact and I am
>>> not sure that for most casual users it is a good trade.
>> So people ARE running their disks in a mode similar to Ramback.
> 
> Similar, but not as aggressive.  Remember, the size of the write cache
> on the hard drive is relatively small (small number of megabytes), and
> the drive generally is relatively aggressive about getting the data
> out to the platters; it's probably not going to keep unwritten data on
> the platters for minutes or hours at a time, let alone days.  Of
> course, unless you use write barriers or some kind of explicit write
> ordering, it's going to write stuff out in an order which is
> convenient to the hard drive, not necessarily an order convenient to
> the filesystem.

You get 8-16MB per disk with most drives today. Different firmware will 
do different things about how aggressively they push the data out to 
platter.

> Also, if the system crashes, you don't lose the data in hard drive's
> write cache, where as the data in Ramback is likely gone.  And Ramback
> is apaprently keeping potentially several gigabytes dirty in memory
> and *not* writing it out very aggressively.  So the exposure is one of
> degree.
> 
> In practice, it's interesting that we've had so few people reporting
> massive data loss despite the lack of the use of write barriers.
> Sure, in absolutely critical situations, it's not a good thing; but if
> I had a mail server, where I really wanted to make sure I didn't lose
> any e-mail, having a small UPS which could keep the server going for
> just a few minutes so it could do a controlled shutdown on a power
> failure is probably a better engineering solution from a
> cost/benefit/performance point of view, compared to turning on write
> barriers and taking up to two orders of magnitude worth of performance
> hit.
> 
> 						- Ted

Most people don't see power outages too often - maybe once a year? When 
you travel with a laptop, we are always effectively on a UPS so that 
will also tend to mask this issue.

The ingest rate at the time of a power hit makes a huge difference as 
well - basically, pulling the power cord when a box is idle is normally 
not harmful. Try that when you are really pounding on the disks and you 
will see corruptions a plenty without barriers ;-)

One note - the barrier hit for apps that use fsync() is just half an 
order of magnitude (say 35 files/sec instead of 120 files/sec). If you 
don't fsync() each file, the impact is lower still.

Still expensive, but might be reasonable for home users on a box with 
family photos, etc.

ric
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