Messages in this thread | | | From | Daniel Phillips <> | Subject | Re: [PATCH] 2.4.x write barriers (updated for ext3) | Date | Tue, 5 Mar 2002 23:56:57 +0100 |
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On March 5, 2002 08:09 am, Jeremy Higdon wrote: > On Mar 4, 8:57am, Daniel Phillips wrote: > > > > On March 4, 2002 07:09 am, Jeremy Higdon wrote: > > > On Mar 4, 6:31am, Daniel Phillips wrote: > > > > On March 4, 2002 05:21 am, Jeremy Higdon wrote: > > > > > I have never heard of > > > > > any implied requirement to flush to media when a drive receives an > > > > > ordered tag and WCE is set. It does seem like a useful feature to have > > > > > in the standard, but I don't think it's there. > > > > > > > > It seems to be pretty strongly implied that things should work that way. > > > > What is the use of being sure the write with the ordered tag is on media > > > > if you're not sure about the writes that were supposedly supposed to > > > > precede it? Spelling this out would indeed be helpful. > > > > > > WCE==1 and ordered tag means that the data for previous commands is in > > > the drive buffer before the data for the ordered tag is in the drive > > > buffer. > > > > Right, and what we're talking about is going further and requiring that WCE=0 > > and ordered tag means the data for previous commands is *not* in the buffer, > > i.e., on the platter, which is the only interpretation that makes sense. > > Sorry to be slow here, but if WCE=0, then commands aren't complete until > data is on the media,
Sorry, I meant FUA, not WCE. For this error I offer the apology that there is a whole new set of TLA's to learn here, and I started yesterday.
> so since previous commands don't complete until > data is on the media, and they must complete before the ordered tag > command does, what you say would have to be the case. I thought the idea > was to buffer commands to drive memory (so that the drive could increase > performance by writing back to back commands without losing a rev) and > then issue a command with a "flush" side effect. > > Here is an interesting question. If you use WCE=1 and then send an > ordered tag with FUA=1, does that imply that data from previous > write commands is flushed to media? I don't think so, though it > would be a useful feature if it did.
That's my point all right. And what I tried to say is, it's useless to have it otherwise, so we should now start beating up drive makers to do it this way (I don't think they'll need a lot of convincing actually) and we should write a test procedure to determine which drives do it correctly, according to our definition of correctness. If we agree on what is correct of course.
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