6 Feb
2021
6 Feb
'21
8:06 a.m.
On Fri, Feb 5, 2021 at 7:04 PM Bakul Shah <bakul(a)iitbombay.org> wrote:
On Feb 5, 2021, at 6:18 AM, Larry McVoy
<lm(a)mcvoy.com> wrote:
The entire point of the SunOS 4.0 VM system was that the page you
saw via mmap(2) is the exact same page you saw via read(2). It's
the page cache, it has page sized chunks of memory that cache
file,offset pairs.
There is one, and only one, copy of the truth. Doesn't matter how
you get at it, there is only one "it".
ZFS broke that contract and that was a step backwards in terms of
OS design.
Let me repeat a part of my response you cut out:
And you can keep track of mapped pages and read/write from them if
necessary even if you have a separate cache for any compressed pages.
In essence you pass the ownership of a page's data from a compressed
page cache to the mapped page. Just like in processor cache coherence
algorithms there is one source of truth: the current owner of a cached
unit (line or page or whatever). In other words, the you see via mmap(2)
will be the exact same page you will see via read(2). Not having actually
tried this I may have missed corner cases + any practical considerations
complicating things but *conceptually* this doesn't seem hard.
In essence, that's what the merged page/buffer cache is all about: file IO
(read/write) operations are satisfied from the same memory cache that backs
up VM objects. I agree that conceptually it's not that complex; but that's
not what ZFS does.
Of course the original Unix buffer cache didn't do that either, because no
one was mmap'ing files on the PDP-11, let alone the PDP-7. A RAM-based
buffer cache for blocks as the nexus around which the system serialized
access to the disc-resident filesystem sufficed. When virtual address
spaces got bigger (starting on the VAX) and folks wanted to start being
more clever with marrying virtual memory and IO, you had an impedance
mismatch with a fairly large extant kernel that had developed not taking
into consideration memory-mapped IO to/from files. Sun fixed this, at what
I take to be great expense (I followed keenly the same path of development
in *BSD and Linux at the time and saw how long that took, so I believe
this). But then the same Sun broke it again.
Warner mentions not using ZFS for its double copying. May be omething
On Thu, Feb 04, 2021 at 09:17:54PM -0800, Bakul Shah wrote:
> On Feb 4, 2021, at 4:33 PM, Larry McVoy <lm(a)mcvoy.com> wrote:
>>
>> Ignoring the page cache and make their own cache has big problems.
>> You can mmap() ZFS files and doing so means that when a page is
referenced
it is copied from the ZFS cache to the page cache.
That creates a
coherency problem, I can write via the mapping and I can write via
write(2) and now you have two copies of the data that don't match,
that's pretty much OS no-no #1.
Write(2)ing to a mapped page sounds pretty dodgy. Likely to get you
in trouble in any case. Similarly read(2)ing. like the above can a step in the direction of
integrating the caches?
But the cache was integrated! Until it wasn't again....
As Ron says, I too would like to hear what the authors of ZFS have to
say....
Sounds like they thought it was too hard because compression means the
place on storage where an offset in a file lands is no longer a linear
function of the file's contents. Presumably the compressed contents are not
kept in RAM in any of the caches (aside from a temporary buffer to which
the compressed contents are read or something).
- Dan C.