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

On Thu, Feb 25, 2016 at 10:02 PM, Dan Williams <dan.j.williams(a)intel.com&gt; 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.

[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 _______________________________________________ Linux-nvdimm mailing list Linux-nvdimm(a)lists.01.org https://lists.01.org/mailman/listinfo/linux-nvdimm