[rsyslog] Development of failsafe disk based queue

[Rainer Gerhards]

On Wed, 2008-10-01 at 14:45 +0200, David Ecker wrote:

[snip]

> as long as you do sector based writes (512 byte per sector, usual) you
> can be sure that the write wasn"t partial.. Writing more than one sector
> or not starting at a correct offset (n*512,n=0,1,2,...x) might result in
> a partial write. I'll already tested that with my devel client here. So
> fencing each sector with a crc32 value would help detecting errors
> during a write operation. This is actually only a problem if you are
> writing directly to a block device like any filesystem does and yes,
> reordering is definitly a problem. So validating the content written to
> the disk afterwards is important.
> 
> If writing through a filesystem reserving space in the destination file
> beforehand actually minimizes errors since the file system table doesn't
> have to be updated (you should also use the Flag O_NOATIME for that
> case). See for example VMWare ESX VMDK file handling.

Well, first of all let me re-iterate that I do not intend to do a block
device driver for rsyslog (but I definitely do not object getting one
contributed ;)).

Still thinking about the case and thinking about non-solid-state,
non-internal-battery-backed-up disk, I can't see how you can be sure the
data will be written. David just told me there are no capacitors. So if
power fails, it fails rather quickly. So how can you be sure the disk
will be able to finish writing that sector? Let's say the drive has
begun to write the sector and been able to write the first 5 bytes. Now
power fails. No capacitors, no battery-backup, so why should there be
enough power to drive the disk write head for another 507 bytes? It the
drives assures it can do that, it needs capacitors - doesn't it?

Am I overlooking something obvious?

Rainer