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Update Tue, 25 Feb 2014 13:12:01 -0800: added "Larger Writes" section.

Update Sat, 01 Mar 2014 19:31:34 -0800: updated "Larger Writes" section with big graph, updated microbenchmark section to link to improved benchmark code.

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I'm optimizing a system in which synchronous disk writes are a key factor in performance. My current drives are too slow. I have a single Intel 520 drive that performs well enough (<1ms), but I need five drives. I bought five Intel 530 SSDs in hopes they would perform like the Intel 520, but instead they take 10ms to write a single byte to disk under various versions of Linux. Curiously, if I connect the Intel 530 drives over a USB-to-SATA adapter instead of using SATA directly, they're much faster (~200us per write). If I turn off write-caching, on some machines they take 1.6ms per write. What's wrong?

Microbenchmark

Here's the original microbenchmark, which calls write() and fdatasync() on a single byte of data 1000 times: https://gist.github.com/ongardie/9177853 I run this as "time ./bench" and divide the wall time by 1000 to get the approximate average time per write.

A much improved and extended version of the benchmark is at https://github.com/ongardie/diskbenchmark .

I normally run this on ext4. (I've tried it on a raw device as well; see negative results section below).A mu

Machines

These are the machines I've tried. The Intel 530 over SATA takes 10ms per write on each of these, and other SSDs take <1ms per write on each of these.

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These are the disks I've tried and their performance:

My goal is to get the Intel 530 drives to run fast on rc66 and similar machines.

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