Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT)

[Avi Kivity <avi@qumranet.com>]

Anthony Liguori wrote:
>
> "cached" is not a terribly accurate term.  O_DIRECT avoids the host 
> page cache but it doesn't guarantee that the disk is using 
> write-through.  For that, you need to use hdparm.
>
> O_SYNC basically turns the host page cache into a write-through 
> cache.  In terms of data integrity, the only question that matters is 
> whether you're misleading the guest into thinking data is on the disk 
> when it isn't.  Both O_DIRECT and O_SYNC accomplish this.
>
> If you just are concerned with data integrity, O_SYNC is probably 
> better because you get the benefits of host caching.  O_DIRECT is 
> really for circumstances where you know that using the host page cache 
> is going to reduce performance.

In one specific circumstance O_SYNC has data integrity problems: shared 
disks with guests running on different hosts (or even a guest on one 
host, sharing a disk with another host).  In these cases, two reads can 
return different values without an intervening write.

In the general case, O_DIRECT gives better performance.  It avoids 
copying from the host pagecache to guest memory, and if you have spare 
memory to benefit from caching, give it to the guest; the nearer to the 
data consumer the cache is, the faster it performs.

In one specific case O_SYNC (or regular read/write cached operation) is 
better, rebooting the same guest over and over, as the guest cache is 
flushed on reboot.  Not a very interesting case, but unfortunately one 
that is very visible.

(if you have a backing file for a COW disk, then opening the backing 
file without O_DIRECT may be a good idea too, as the file can be shared 
among many guests).

-- 
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