RAM disk vs paging + buffer cache
Geoff Kuenning
geoff at desint.UUCP
Thu Aug 14 17:01:20 AEST 1986
In article <240 at whuxcc.UUCP> judah at whuxcc.UUCP (Judah Greenblatt) writes:
> I'm not
> sure why, but it seems that the following sequence generates several
> physical I/O operations, even when the buffer-cache is empty:
> - create a file
> - write 1 block of data
> - close it
This is an unfortunate side effect of the file system reliability
enhancements that were done for System V (III?). This is the unfortunate
reality of reliability--it trades off against performance. In this case,
whenever an "important" change is made to an i-node, it is written to
disk immediately. I believe this also applies to directories. It has
a negative effect on performance in several ways, but most users seem
to feel the reliability is worth it.
> - One thought on why you might not want to let the block buffers
> do all the work: can you imagine what a 'sync' would
> cost on a system with 20,000 block buffers?
Also, can you imagine what it would be like to crash WITHOUT sync-ing on a
system with 20,000 block buffers? Even with the reliabiltiy enhancements?
--
Geoff Kuenning
{hplabs,ihnp4}!trwrb!desint!geoff
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