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How do I enable write caching in linux?
I'm running a server that is constantly writing to disk and thrashing the disks. I have a feeling that if I could allocate 1GB of RAM to a write cache all my i/o problems would improve. But from what I understand the linux kernel disabled write caching and I don't know the first thing about enabling this feature. And to top it off everyone I talk to about this subject is neurotic about enabling write cache and giving me horror scenarious about data loss. I don't care! I want write cache so how do I get it?
Thanks for the tips but what I want to do is make sure that thousands of tiny write requests on massive fragmented files on my Xen VPS node does not crash the box due to the fragmentation issues along with a gazillion tiny commits to massive log files. So since Xen uses LVs to do it's writing on I need to take care of this at the kernel/driver level
You are confusing two different types of write caching. Linux does use huge amounts of memory as a write cache.
The type of write caching that you are hearing horror stories about is write caching in the drive itself. Linux carefully orders the write requests from its write cache to the drive, but if the drive internally caches writes, this security can be defeated and data loss can occur.
You can tune the way Linux does write caching. However, I strongly urge you not to make the cache too big. Ultimately, if the computer is writing faster than the drive can write, however big your cache is, it will fill up. At that point, the computer will need to wait for space in the cache anyway. The larger cache will likely just mean more data lost in the event of a crash and it will mean less RAM for other things, like read caching.
Tuning your filesystem may have more of a payoff anyway.
Update: A big cache won't really help. If the writes are faster than the drive, they will *have* to block until the drive is ready. Cached data would grow to fill a cache of any size. The cache needs to be just big enough to smooth out the 'bursts' on both ends.
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Enable Write Caching on the Primary Hard Drive | Faster 7 Part 12
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