Messages in this thread | | | Date | Fri, 14 Mar 2008 11:47:04 -0400 | From | Ric Wheeler <> | Subject | Re: [ANNOUNCE] Ramback: faster than a speeding bullet |
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Theodore Tso wrote: > On Fri, Mar 14, 2008 at 12:41:31PM +0100, Benny Amorsen wrote: >> Ric Wheeler <ric@emc.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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