Swapping to MMC (Was: To swap or not to swap)

RW fbsd06 at mlists.homeunix.com
Tue Dec 2 14:57:46 PST 2008

On Tue, 2 Dec 2008 20:47:26 +0000
"Anthony M. Rasat" <anthony.rasat at gmail.com> wrote:

> Pros: 1) System requires swap. Period. 2) Swap may need size in range
> between 2.17 times to 2.22 time or whatever size it need. This is not
> prohibited by Eee's SSD size (4GB btw, 701 series).

Add what swap you need, but in my experience things get pretty slow
before you even reach 1x system memory on hard disks. AFAIK the 2x
figure is to do with saving kernel dumps, which you probably don't want
to bother with.

> Cons: 1) Since SSD is manufactured have limited lifetime (around
> 100,000 times write operation or so, I read it somewhere), swapping
> to SSD is more likely not a wise thing to do.

I don't think it's much of a problem with modern wear-levelling.
It's 100,000 writes per block with the writes being spread evenly over
the device (albeit with extra write for the wear-levelling). 

There are few writes to swap until you run low on memory, so simply
having swap wont by itself wear out the device.

> Two against one. I concurr that swap is needed. However since SSD in
> 701 series is not removable, having a bad sector in SSD is one thing
> you don't want to have.

I would think they have spare sectors like hard disks do.

> performances? And what happened if FreeBSD kernel suddenly lose its
> swap file by absent-minded human? Is it going to be just angry or
> having massive heart attack?

I'm not sure whether an active swap file can be deleted or not, but it
would be owned by root, so not deletable by a normal user. I suspect
that it would behave like an open file and not be genuinely deleted
until swapoff'ed. In any case it's not more of a risk than deleting any
critical file.

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