Disable read/write caching to disk?
Eric Anderson
anderson at centtech.com
Thu May 26 11:12:53 PDT 2005
Scott Long wrote:
> Eric Anderson wrote:
>
>> Bjoern Koenig wrote:
>>
>>> Bjoern Koenig wrote:
>>>
>>>> Eric Anderson wrote:
>>>>
>>>>> Is it possible to disable all read and write caching to a disk?
>>>>
>>>>
>>>>
>>>>
>>>> You can disable write the cache by adding the line
>>>>
>>>> hw.ata.wc="0"
>>>
>>>
>>>
>>>
>>> I assumed that you use ATA. If you use SCSI devices then read at
>>> least the manpages da(4) and camcontrol(8).
>>
>>
>>
>> Thanks.. I've just read (quickly) both man pages. It seems as though
>> you are suggesting disabling the physical disk caching, which should
>> not make a difference in my case. The disk would report whatever it
>> needs to report to either host, and those should be in sync.
>>
>> When I mount the filesystem on host B ro, it shows me the filesystem
>> as of the time that I mounted it ro. Any subsequent changes on host A
>> (which has it mounted rw) are not seem on host B unless I unmount and
>> mount again on host B. This seems like a FreeBSD feature and not a
>> general scsi feature.
>>
>> Eric
>>
>>
>>
>>
>
> You simply cannot disable OS caching in FreeBSD. It's a fundamental
> part of the block I/O and VM layers. There are filesystems like GFS
> that deal with the issue of directly connecting more than one computer
> to a disk or set of disks, and there are distributed filesystems like
> AFS and Coda that deal with making the storage on multiple computers
> appear as a single network filesystem. Unfortunately, no port of GFS
> has been done yet, and I estimate that such a port would take 4-6 months.
Thanks Scott. I know of GFS, and would *love* to see it ported to
FreeBSD. I wonder if there is a group of developers that would be
willing to do that?
I understand what I am doing is 'illegal', but I'm wondering why the ro
mount only sees the changes from the time of ro mount. Mounting rw also
shows the same thing.
Do you think it's a caching issue, or something with UFS that makes it
work this way?
I'm in no way advocating doing this, nor am I saying 'it should work' -
I'm trying to learn more, understand it, and maybe use it as a failover
mechanism.
Does anyone know the real dangers of forcing an unclean UFS filesystem
mounted rw and skipping the fsck?
Eric
--
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Eric Anderson Sr. Systems Administrator Centaur Technology
A lost ounce of gold may be found, a lost moment of time never.
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