[mythtv-users] Contemplating a major change

[Raymond Wagner]

On 4/6/2014 1:46 PM, Stephen P. Villano wrote:
> On 4/6/14, 2:24 PM, Gary Buhrmaster wrote:
>> On Sun, Apr 6, 2014 at 6:05 PM, Stephen P. Villano
>> <stephen.p.villano at gmail.com> wrote:
>>> On 4/6/14, 12:49 PM, Raymond Wagner wrote:
>>>> The "hardware" part was key.  The only thing hardware RAID10 gives you
>>>> over software is ease of boot.  Hardware RAID is only beneficial when
>>>> you have to perform parity calculations, and even then it's largely
>>>> inconsequential on a modern CPU.
>>> For RAID10, perhaps, perhaps not. It's used in enterprise for a good reason.
>> And many of those enterprise controllers also have various
>> battery backed caches.  Which *can* have benefit for
>> certain workloads (often databases), although various
>> SSD devices are now the usual accelerator since the
>> logs tend to be small enough (even a ZIL on a large
>> ZFS installation can often fit in a reasonably priced
>> (for enterprise quality) SSDs), and the L2ARC is also
>> moving to SSDs for more performant target installations.
> I don't know. Logs are *usually* small, but when something goes wrong,
> well, my Mythbuntu 12.04 ended with five gigs of errors and climbing
> once, with a backup of 5 gigs of compressed log.
> Logs are typically small until a system error occurs, then the sky's the
> limit!

Different log.  The ZIL or ZFS Intent Log is what ZFS "intends" to write 
to disk.  It fulfills the same purpose as the battery-backed write cache 
on hardware RAID cards.  You write to the non-volatile ZIL nearly 
instantly, and then ZFS takes care of pushing that data to rotating 
storage when convenient.  The general rule of thumb for a ZIL is roughly 
10 seconds at your maximum write throughput, so a few GB for a large 
RAID array.  When using ZFS with a ZIL, you typically partition just a 
small amount of it for ZIL, and leave the rest for L2ARC, which is a 
second level read cache, stacked on top of the typical in-memory disk cache.