gjournal and calculation of the size of journal provider

[Ivan Voras]

Miroslav Lachman wrote:
> Ivan Voras wrote:
>> Miroslav Lachman wrote:
>>> What is the right rule for journal size calculation?
>>> There are two sources stating different things.
>>>
>>> 1] journal size depends on disk write speed
>>> http://lists.freebsd.org/pipermail/freebsd-fs/2006-June/002016.html
>>>
>>> "For example your disk can write
>>> at 60MB/s. Journal switch time is 10 seconds. The journal provider has
>>> to have place to keep two journals (active and inactive). So bascially
>>> you need 60*10*2MB + gjournal headers."
>>>
>>>
>>> 2] journal size depends on RAM size
>>> http://www.freebsd.org/doc/en/articles/gjournal-desktop/article.html#UNDERSTANDING-JOURNALING 
>>>
>>>
>>>
>>> "Your RAM size should fit in 30% of the journal provider's space. For
>>> example, if your system has 1 GB RAM, create an approximately 3.3 GB
>>> journal provider. (Multiply your RAM size with 3.3 to obtain the size
>>> of the journal)."
>>>
>>>
>>> What's the right size for journal on 143GB 15k rpm SAS disks on
>>> machine with 16GB of RAM? Based on second case, it will be more than
>>> 50 GB - one third of the size of disk. This is insane vasting.
>>
>> It really does depend on the speed of drives but it could be
>> approximated by saying there will not be more data to write than the
>> size of memory (which is probably wrong since you can write from
>> /dev/zero indefinitely). The first advice is sufficient, but you should
>> probably extend the result by 20% to be safer.
> 
> 
> So is it safe to use 4GB on PERC6 array, which is capable of 150MB/s 
> write speed by dd test?
> dd if=/dev/zero of=/dev/mfid0s2e bs=1m count=10000
> 
> (150 * 10 * 2 * 1.2) = 3600
> 
> 150 is write speed in MB/s
> 10 is journal switch time
> 2 is active + inactive journal
> 1.2 is +20% to base safer

This looks fine!

> And next question about journal. I saw following message in log after 
> reboot:
> 
> GEOM_JOURNAL: Journal 1933335573: mfid0s2d contains journal.
> GEOM_JOURNAL: Journal 1933335573: mfid0s2e contains data.
> GEOM_JOURNAL: Journal mfid0s2e clean.
> GEOM_JOURNAL: BIO_FLUSH not supported by mfid0s2d.
> GEOM_JOURNAL: BIO_FLUSH not supported by mfid0s2e.
> 
> "BIO_FLUSH not supported" - is it OK to use gjournal on top of the Dell 
> PERC (LSI MegaRAID) with battery backup unit? I think so, but rather ask 
> somebody... :)

I think you are safe if the controller has the BBU working and enabled.