On 8/25/19 5:03 PM, Nir Soffer wrote: > When creating an image with preallocation "off" or "falloc", the first > block of the image is typically not allocated. When using Gluster > storage backed by XFS filesystem, reading this block using direct I/O > succeeds regardless of request length, fooling alignment detection. > > In this case we fallback to a safe value (4096) instead of the optimal > value (512), which may lead to unneeded data copying when aligning > requests. Allocating the first block avoids the fallback. >
> Here is a table comparing the total time spent:
>
> Type Before(s) After(s) Diff(%)
> ---------------------------------------
> real 530.028 469.123 -11.4
> user 17.204 10.768 -37.4
> sys 17.881 7.011 -60.7
>
> We can see very clear improvement in CPU usage.
Nice justification.
> +/*
> + * Help alignment probing by allocating the first block.
> + *
> + do {
> + n = pwrite(fd, buf, write_size, 0);
> + } while (n == -1 && errno == EINTR);
> +
> + qemu_vfree(buf);
qemu_vfree() can corrupt errno...
> +
> + return (n == -1) ? -errno : 0;
...which means you may be returning an unexpected value here.
Either we should patch qemu_vfree() to guarantee that errno is
preserved, or you locally capture errno before calling it here.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org
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