Disable read/write caching to disk?

[Scott Long]

Eric Anderson wrote:

> 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'd love to do it sometime.  Some diligence would have to be done on the
license and possible patents, though.  I'm not sure what RedHat's stance
is on sharing in this regard.

> 
> 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?
> 

Even on a read-only mount, reads are cached.  Cached blocks are only
evicted when there is memory pressure or when the OS specifically
invalidates them, and it's rather unpredictable when this will happen.

> 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.

The only thing that is moderately workable is to have all client
machines mount the FS read-only, and have none of them do any writing
to it.

> 
> Does anyone know the real dangers of forcing an unclean UFS filesystem 
> mounted rw and skipping the fsck?

Assuming that the previous shutdown allowed all cached metadata in the 
disk to get to the platters in the proper order (which is not terribly 
true with ATA), the main inconsistency that you'll have is that deleted 
files might still have allocated inodes and thus will consume space on 
the filesystem even though they aren't accessible.  This is the premise 
that background fsck operates on, btw.

However, there is a real possibility for there being other 
inconsistencies that are not safe to run with.

Scott