[asterisk-users] Asterisk RT on Disk On Module Performance and Durability

[Kristian Kielhofner]

Jeremy McNamara wrote:
> Tzafrir Cohen wrote:
>  > Hmmmm, I'm not sure that this is exactly the data you're after.
>>
>> You're looking for the ammounts of writes for the disk block that gets
>> the most writes.
>>
>> E.g: for a standard ext3 filesystem, the journal area would probably
>> have very frequent writes, whereas most of the system would remain
>> mostly unchanged.
> 
> 
> Again, if the embedded system is setup properly, there is NO writing to 
> the flash during normal operations, thus the device won't be killed by 
> its alleged 2 million write limitation.
> 
> Kris and I had a quick discussion on this topic, off-list, and his 
> original flash-based device is still in constant operation after 2 years 
> and I have flash modules that I purposely tried to kill with writes. It 
> took significant effort to start causing error situations, which were 
> very easily detected before the system would become unusable.
> 
> Erick, you should focus on having a quick action restoration plan and 
> extra DOMs always readily available.  Then when a failure situation is 
> detected, you can react very quickly.
> 
> 
> 
> 
> Jeremy McNamara

Jeremy, Erick -

	I have always pointed to this SanDisk whitepaper:

http://www.sandisk.com/Assets/File/OEM/WhitePapersAndBrochures/RS-MMC/WPaperWearLevelv1.0.pdf

	While it specifically discusses their industrial line of CF cards, it 
is pretty obvious that flash can, and often does, last much longer than 
other components in a system when properly implemented.  You will notice 
that the SHORTEST expected life of a CF card in their test scenarios was 
over 70 years!  How long is your power supply going to last?  Even if 
the consumer level cards had 1/10 the life expectancy, that is still 
seven years.  I expect to get at least that from my original AstLinux 
system.  It's been two so far, I'll let you know how it is doing in 
another five years :).

	JFFS (and similar FSs) are not appropriate for CF cards or DOMs.  They 
are meant to be used directly on flash memory and do their own wear 
leveling and in some cases, compression.  All kinds of commercial 
devices use JFFS2.  If you are using a CF or DOM with Linux, ext2 is the 
best FS to use.  CF cards and DOMs use their own wear leveling, so none 
is required in the operating system or file system.  CF cards and DOMs 
hide wear leveling from you and expose themselves as an ordinary IDE device.

	I echo Jeremy's conclusions.  With a properly designed operating 
system, decent flash memory, and a reasonable usage pattern, I can tell 
you (with a great amount of certainty) that in most situations, CF cards 
will outlast just about any hard drive (even SCSI) when used 24/7. 
These days, it really is pretty tough to trash flash.

	However, if you are running a MySQL cluster or something with several, 
multi-gigabyte databases, no type of flash memory will last very long! :)

	To get back to answering your question, I HIGHLY recommend that you 
avoid MySQL and realtime on your box with a DOM.  Nothing against either 
(MySQL or Realtime), but they will probably make your device more 
complicated than it needs to be while substantially shortening the life 
of your DOM.  If you absolutely have to use MySQL, you might have better 
luck using a MySQL storage engine that uses fewer writes than InnoDB, 
but I am no expert on that.

--
Kristian Kielhofner