Messages in this thread | | | Date | Fri, 14 Mar 2008 08:56:56 -0400 | From | Theodore Tso <> | Subject | Re: [ANNOUNCE] Ramback: faster than a speeding bullet |
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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.
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
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