Re: swap file over jffs2 partition

Re: swap file over jffs2 partition

[Jörn Engel]

On Fri, 29 February 2008 04:50:17 +0200, Dimitrios Apostolou wrote:
> 
> I intend to build a diskless linux system (root over NFS). Because it
> has 1GB of embedded flash storage, I'm thinking of using this as swap 
> (I've been bitten many times by the problems linux has with *no* 
> swap...). And to avoid wearing out the flash storage too fast, I 'm 
> thinking to format the 1GB partition as JFFS2, and create the swapfile 
> on top of it.
> 
> I'm not so experienced with JFFS and I don't know if it's too heavy for
> the CPU, for swapping. Or if there are other issues I 'll face. What do
> you think about it? Any other ways you 'd propose?
> 
> Sorry for sending this at LKML but jffs-dev mailing list seems to be 
> off. And JFFS is the only in-kernel filesystem that does wear-leveling, 
> right?

Replying in reverse order...

The relevant mailing list is linux-mtd, added to Cc:.  JFFS and JFFS2
are two different things, JFFS is older and was removed from the kernel
not too long ago.

The real fun comes not from CPU usage, but from interactions with the
memory management subsystem.  In a nutshell, JFFS2 may require memory in
order to write data.  When the system is under memory pressure, it needs
JFFS2 to write out pages, which will try to allocate memory.  It is
theoretically possible to deadlock the system in this way.

On the plus side, the write path of JFFS2 is relatively simple and
extremely low-latency.  It shouldn't be too hard to review the code and
handle all problem cases wrt. memory allocations.

One issue that is hard to solve is space reservation.  JFFS2 compresses
data and allows users to write as long as there is space remaining.  It
is possible to swap out data that compresses well, have some other
process fill up the filesystem, then try to swap out data that
compresses badly and get -ENOSPC in return.  As a system administrator
you can prevent others from ever writing to JFFS2 - and you better do!

Jörn

-- 
Joern's library part 5:
http://www.faqs.org/faqs/compression-faq/part2/section-9.html

Re: swap file over jffs2 partition

[Dimitrios Apostolou]

Jörn Engel wrote:
> On Fri, 29 February 2008 04:50:17 +0200, Dimitrios Apostolou wrote:
>> I intend to build a diskless linux system (root over NFS). Because it
>> has 1GB of embedded flash storage, I'm thinking of using this as swap 
>> (I've been bitten many times by the problems linux has with *no* 
>> swap...). And to avoid wearing out the flash storage too fast, I 'm 
>> thinking to format the 1GB partition as JFFS2, and create the swapfile 
>> on top of it.
>>
>> I'm not so experienced with JFFS and I don't know if it's too heavy for
>> the CPU, for swapping. Or if there are other issues I 'll face. What do
>> you think about it? Any other ways you 'd propose?
>>
>> Sorry for sending this at LKML but jffs-dev mailing list seems to be 
>> off. And JFFS is the only in-kernel filesystem that does wear-leveling, 
>> right?
> 
> Replying in reverse order...
> 
> The relevant mailing list is linux-mtd, added to Cc:.  JFFS and JFFS2
> are two different things, JFFS is older and was removed from the kernel
> not too long ago.

Thanks and sorry for intruding LKML. It seems that even wikipedia has 
wrong address for the mailing list, see the last link of the article:

http://en.wikipedia.org/wiki/JFFS2

> 
> The real fun comes not from CPU usage, but from interactions with the
> memory management subsystem.  In a nutshell, JFFS2 may require memory in
> order to write data.  When the system is under memory pressure, it needs
> JFFS2 to write out pages, which will try to allocate memory.  It is
> theoretically possible to deadlock the system in this way.

Interesting. I guess nobody has experimented with it yet so I'll try. 
Unfortunately it seems I'll face another problem, that JFFS2 doesn't 
support having a swap-file at all. Why would this happen? More info:

http://dev.laptop.org/ticket/6469

> 
> On the plus side, the write path of JFFS2 is relatively simple and
> extremely low-latency.  It shouldn't be too hard to review the code and
> handle all problem cases wrt. memory allocations.
> 
> One issue that is hard to solve is space reservation.  JFFS2 compresses
> data and allows users to write as long as there is space remaining.  It
> is possible to swap out data that compresses well, have some other
> process fill up the filesystem, then try to swap out data that
> compresses badly and get -ENOSPC in return.  As a system administrator
> you can prevent others from ever writing to JFFS2 - and you better do!

Of course! I intend to use all the 1GB of flash only for swap, the 
system will be practically diskless. And I don't think enabling 
compression for such a task would be wise.

> 
> Jörn
> 

Thanks for the help,
Dimitris

Re: swap file over jffs2 partition

[Jörn Engel]

On Fri, 29 February 2008 19:29:53 +0200, Dimitrios Apostolou wrote:
> 
> Thanks and sorry for intruding LKML. It seems that even wikipedia has 
> wrong address for the mailing list, see the last link of the article:
> 
> http://en.wikipedia.org/wiki/JFFS2

Fixed.  Not that I actually read the page and checked for other
errors...

> Interesting. I guess nobody has experimented with it yet so I'll try. 
> Unfortunately it seems I'll face another problem, that JFFS2 doesn't 
> support having a swap-file at all. Why would this happen? More info:
> 
> http://dev.laptop.org/ticket/6469

Yes, this still requires a bit of work.  Peter Zijlstra has a patch for
the memory-management side of it, someone has to fix up jffs2 as well.
I'll add support to logfs fairly soon.

> Of course! I intend to use all the 1GB of flash only for swap, the 
> system will be practically diskless. And I don't think enabling 
> compression for such a task would be wise.

It might, actually.  Assuming your average swap page compresses to 50%,
it means that you use half the bandwidth to flash and get twice the
lifetime before blocks wear out.  Whether the performance gets better or
worse depends on the relative speeds of your CPU and flash.

Jörn

-- 
Joern's library part 10:
http://blogs.msdn.com/David_Gristwood/archive/2004/06/24/164849.aspx