On Wed, 18 Dec 2013 21:50:00 +1100, Chris Angelico <ros...@gmail.com>
wrote:
>On Wed, Dec 18, 2013 at 9:31 PM, Cameron Simpson <c...@zip.com.au> wrote:
>> On 18Dec2013 14:35, Chris Angelico <ros...@gmail.com> wrote:
>>> An SQL database *is* a different form of storage. It's storing tabular
>>> data, not a stream of bytes in a file. You're supposed to be able to
>>> treat it as an efficient way to locate a particular tuple based on a
>>> set of rules, not a different way to format a file on the disk.
>>
>> Shrug. It's all just data to me. I don't _care_ about the particular
>> internal storage format.
>
>Then use a file, because you want file semantics. That's why you have
>both options available.
>
>> Commit() is a logical operation saying this SQL changeset is now
>> part of the global state.
>
>The global state is defined by what's on the disk. Specifically, by
>what would be read if the power failed right at that moment. In the
>case of PostgreSQL, a commit doesn't actually write the table pages -
>it just writes the WAL (Write-Ahead Log), which is used to recreate
>the transaction. If something fails hard, the WAL replay will apply
>the change perfectly. That's the global state. It's not there till the
>WAL's been fsync'd.
>
<snip>
>
>> All that has happened after an fsync() is that the OS taken your
>> SQL changeset that you commited to the OS data abstraction and
>> pushed it one layer lower into the "disk" abstraction. There's more
>> going on in there.
>
>Not just pushed it one layer lower; the point of fsync is that it's
>been pushed all the way down. See its man page [1]:
>
>"""fsync() transfers ("flushes") all modified in-core data ... to the
>disk ... so that all changed information can be retrieved even after
>the system crashed or was rebooted."""
>
>It's fundamentally about crash recovery, not about "passing it to a
>lower abstraction". Of course, the OS isn't always *able* to guarantee
>things (NFS shares are notoriously hard to pin down), but the
>intention of fsync is that it won't return (and therefore the COMMIT
>operation won't finish) until the data can be read back reliably even
>in the event of a major failure.
>
>Databases protect against that. If you want that protection, use a
>database. If you don't, use a file. There's nothing wrong with either
>option.
>
>ChrisA
>
>[1] on the web here, for those who don't have them handy:
>http://linux.die.net/man/2/fsync
Don't forget that most hard disks have an option to cache the write
data. This is a 'feature' that allows the manufacturers to claim
better write performance. You can't be sure when the data is written
to the disk if that option is in play.
Dick
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