Re: [RFC 0/2] New MAP_PMEM_AWARE mmap flag

Re: [RFC 0/2] New MAP_PMEM_AWARE mmap flag

[Boaz Harrosh]

On 02/26/2016 12:04 PM, Thanumalayan Sankaranarayana Pillai wrote:
> On Thu, Feb 25, 2016 at 10:02 PM, Dan Williams <dan.j.williams@intel.com> wrote:
>> [ adding Thanu ]
>>
>>> Very few applications actually care about atomic sector writes.
>>> Databases are probably the only class of application that really do
>>> care about both single sector and multi-sector atomic write
>>> behaviour, and many of them can be configured to assume single
>>> sector writes can be torn.
>>>
>>> Torn user data writes have always been possible, and so pmem does
>>> not introduce any new semantics that applications have to handle.
>>>
> 
> I know about BTT and DAX only at a conceptual level and hence do not understand
> this mailing thread fully. But I can provide examples of important applications
> expecting atomicity at a 512B or a smaller granularity. Here is a list:
> 
> (1) LMDB [1] that Dan mentioned, which expects "linear writes" (i.e., don't
> need atomicity, but need the first byte to be written before the second byte)
> 
> (2) PostgreSQL expects atomicity [2]
> 
> (3) SQLite depends on linear writes [3] (we were unable to find these
> dependencies during our testing, however). Also, PSOW in SQLite is not relevant
> to this discussion as I understand it; PSOW deals with corruption of data
> *around* the actual written bytes.
> 
> (4) We found that ZooKeeper depends on atomicity during our testing, but we did
> not contact the ZooKeeper developers about this. Some details in our paper [4].
> 
> It is tempting to assume that applications do not use the concept of disk
> sectors and deal with only file-system blocks (which are not atomic in
> practice), and take measures to deal with the non-atomic file-system blocks.
> But, in reality, applications seem to assume that 512B (more or less) sectors
> are atomic or linear, and build their consistency mechanisms around that.
> 

This all discussion is a shock to me. where were these guys hiding, under a rock?

In the NFS world you can get not torn sectors but torn words. You may have
reorder of writes, you may have data holes the all deal. Until you get back
a successful sync nothing is guarantied. It is not only a client
crash but also a network breach, and so on. So you never know what can happen.

So are you saying all these applications do not run on NFS?

Thanks
Boaz

> [1] http://www.openldap.org/list~s/openldap-devel/201410/msg00004.html
> [2] http://www.postgresql.org/docs/9.5/static/wal-internals.html , "To deal
> with the case where pg_control is corrupt" ...
> [3] https://www.sqlite.org/atomiccommit.html , "SQLite does always assume that
> a sector write is linear" ...
> [4] http://research.cs.wisc.edu/wind/Publications/alice-osdi14.pdf
> 
> Regards,
> Thanu
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