On 2016-09-23 13:05, David Holland wrote:
On Fri, Sep 23, 2016 at 11:49:50AM +0200, Edgar Fu? wrote: > > The whole point of tagged queueing is to let you *not* set [the write > > cache] bit in the mode pages and still get good performance. > > I don't get that. My understanding was that TCQ allowed the drive > to re-order commands within the bounds described by the tags. With > the write cache disabled, all write commands must hit stable > storage before being reported completed. So what's the point of > tagging with cacheing disabled? You can have more than one in flight at a time. Typically the more you can manage to have pending at once, the better the performance, especially with SSDs.
I'd say especially with rotating rust, but either way... :-)Yes, that's the whole point of tagged queuing. Issue many operations. Let the disk and controller sort out in which order to do them to make it the most efficient.
With rotating rust, the order of operations can make a huge difference in speed. With SSDs you don't have those seek times to begin with, so I would expect the gains to be marginal.
Johnny
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