Re: [gentoo-user] Synchronous writes over the network.

[Rich Freeman]

On Mon, Dec 20, 2021 at 2:52 PM Wols Lists <antlists@××××××××××××.uk> wrote:
>
> And it might also mean blocking writes, which is why you don't want it
> on spinning rust. But it also means that it is (almost) guaranteed to
> get to permanent storage, which is why you do want it for mail,
> databases, etc.
>

The reason that mail/databases/etc use synchronous behavior isn't
because it is "almost" guaranteed to make it to storage.  The reason
they use it is because you have multiple hosts, and each host can
guarantee non-loss of data internally, but synchronous behavior is
necessary to ensure that data is not lost on a handoff.

Take a mail server.  If your SMTP connection goes down for any reason
before the server communicates that the mail was accepted then the
sender will assume the mail was not delivered, and will try again.  So
if the network goes down, or the SMTP server crashes, then the client
will cache the mail and try again.  Most mail servers will have the
data already on-disk before even attempting to deliver mail, so even
if all the computers involved go down during this handoff nothing is
lost as it is still in the client cache on-disk.

On the other hand, once the server confirms delivery then
responsibility is handed off and the client can forget about the mail.
It is important that the mail server not communicate that the mail was
received until it can guarantee that it won't lose the mail.  That is
usually accomplished by the server syncing the mail file to the
on-disk spool and blocking until that is successful before
communicating back to the client that the mail was delivered.

Database transactions behave similarly.

If the userspace application either does a write on a file opened with
O_SYNC or does an fsync system call on the file, and the system call
returns, then the data is present on-disk and will be persistent even
if the power is lost at the very next moment.  It is acceptable for a
filesystem to return the call if the data is in a persistent journal,
which is what the ZIL is, as long as it is flushed to disk.

Of course, you can still accept mail or implement a database
asynchronously, but you lose a number of data protections that are
otherwise designed into the software (well, assuming you're not
storing your data in MyISAM...).  :)

-- 
Rich