How to use a terabyte of RAM [LWN.net]

How to use a terabyte of RAM

Posted Mar 14, 2008 22:07 UTC (Fri) by giraffedata (subscriber, #1954)
In reply to: How to use a terabyte of RAM by dlang
Parent article: How to use a terabyte of RAM

Thanks. Much clearer now.

If I have 500 GB of memory, Violin looks like a much better way than Ramback to use it. The reason is that it better answers my mistrust of the battery, hardware, and Linux. I can put multiple Violins in a RAID array and when the battery, hardware, or Linux fails in one, I don't lose data. And it looks roughly as fast as Ramback.

If I do put 500 GB of memory in the application server, I think I'd just like to add it to the pool and let the memory manager decide if caching all the contents of my filesystems is the best use for it. Maybe with some parameters to say I trust my battery, hardware, and Linux enough that Linux need not hurry to write any of it back to disk.

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How to use a terabyte of RAM

Posted Mar 15, 2008 19:32 UTC (Sat) by daniel (subscriber, #3181) [Link]

"Violin looks like a much better way than Ramback to use it. The reason is that it better
answers my mistrust of the battery, hardware, and Linux. I can put multiple Violins in a RAID
array and when the battery, hardware, or Linux fails in one, I don't lose data. And it looks
roughly as fast as Ramback."

It is exactly as fast as ramback, because it is ramback.  Ramback was written to provide the
Violin box with stable backing store.  It is just a nice bonus that ramback happens to be
useful for ramdisks in general, and for my code hacking workstation in particular.

How to use a terabyte of RAM

Posted Mar 15, 2008 22:30 UTC (Sat) by giraffedata (subscriber, #1954) [Link]

OK, then, based on that and some more reading, I believe the answer to my question, "what is the relationship between Ramback and Violin" is this: Ramback couples a nonpersistent block device (a device that doesn't retain its memory across an orderly shutdown) with persistent storage so as to create a block device with the speed of RAM and the persistence of disk. The violin box is one source of a nonpersistent block device, and the one that inspired Ramback. The box uses DRAM for storage and connects to the application server via PCI express and comes with a driver for Linux to make a Linux block device out of it. Ramback, running on the application server, uses that block device.

That means my earlier comments comparing use of Violin with use of Ramback are nonsense; they aren't alternatives because one provides persistent storage and the other doesn't. And my comparison of using Ramback to adding memory to the regular pool is similarly nonsense because Ramback can use DRAM that isn't in the application server (e.g. the Violin box).