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SubjectRe: [ANNOUNCE] Ramback: faster than a speeding bullet
On Sun, 9 Mar 2008, Daniel Phillips wrote:
> Every little factor of 25 performance increase really helps.
> Ramback is a new virtual device with the ability to back a ramdisk
> by a real disk, obtaining the performance level of a ramdisk but with
> the data durability of a hard disk. To work this magic, ramback needs
> a little help from a UPS. In a typical test, ramback reduced a 25
> second file operation[1] to under one second including sync. Even
> greater gains are possible for seek-intensive applications.
> The difference between ramback and an ordinary ramdisk is: when the
> machine powers down the data does not vanish because it is continuously
> saved to backing store. When line power returns, the backing store
> repopulates the ramdisk while allowing application io to proceed
> concurrently. Once fully populated, a little green light winks on and
> file operations once again run at ramdisk speed.
> So now you can ask some hard questions: what if the power goes out
> completely or the host crashes or something else goes wrong while
> critical data is still in the ramdisk? Easy: use reliable components.
> Don't crash. Measure your UPS window. This is not much to ask in
> order to transform your mild mannered hard disk into a raging superdisk
> able to leap tall benchmarks at a single bound.
> If line power goes out while ramback is running, the UPS kicks in and a
> power management script switches the driver from writeback to
> writethrough mode. Ramback proceeds to save all remaining dirty data
> while forcing each new application write through to backing store
> immediately.
> If UPS power runs out while ramback still holds unflushed dirty data
> then things get ugly. Hopefully a fsck -f will be able to pull
> something useful out of the mess. (This is where you might want to be
> running Ext3.) The name of the game is to install sufficient UPS power
> to get your dirty ramdisk data onto stable storage this time, every
> time.

Are you using barriers or ordered disk writes with physical sync in the
right moments or something like that? I think this is needed to allow any
journaling filesystem to do it's job right.

> The basic design premise of ramback is alluringly simple: each write to
> a ramdisk sets a per-chunk dirty bit. A kernel daemon continuously
> scans for and flushes dirty chunks to backing store.



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