It seems that the rate of spurious I/O errors varies most according to the vm.vfs_cache_pressure sysctl. At '10' the I/O errors occur so often that building a kernel is impossible. At '100' I can't reproduce even a single I/O error. I guess this is own my fault for using non-default sysctl parameters, although I wouldn't expect any value of this sysctl to cause these symptoms... :-P On Sun, Jan 25, 2015 at 11:50:36AM -0500, Zygo Blaxell wrote: > On Sat, Jan 24, 2015 at 01:06:01PM -0500, Zygo Blaxell wrote: > > I am seeing a lot of spurious I/O errors that look like they come from > > the cache-facing side of btrfs. While running a heavy load with some > > extent-sharing (e.g. building 20 Linux kernels at once from source trees > > copied with 'cp -a --reflink=always'), some files will return spurious > > EIO on read. It happens often enough to prevent a Linux kernel build > > about 1/3 of the time. > [...] > > Observed from 3.17..3.18.3. All filesystems affected use skinny-metadata. > > No filesystems that are not using skinny-metadata seem to have this > > problem. > > I ran a test overnight using 3.18.3 on a freshly formatted filesystem with > no skinny-metadata. > > The test consisted of creating reflink copies of a Linux kernel source > tree and running kernel builds in each copy simultaneously, like this: > > # assume you have a ready-to-build kernel tree in 'linux' > for x in $(seq 1 5); do > cp -a --reflink linux linux-$x > done > > # build all the kernels at once > for x in $(seq 1 5); do > (cd linux-$x && make -j10 2>&1 | tee make.log) & > done > > wait > # then tail all the make.logs and see how many failed due to > # I/O errors > > Spurious I/O errors occured with as few as two concurrent kernel builds. > > The test machine has 16GB of RAM and the filesystem is also 16GB, > RAID1 on two spinning disks. >
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