NILFS: A File System to Make SSDs Scream
linux-mag.com
linux-mag.com
One of the most noticeable features of NILFS is that it can “continuously and automatically save instantaneous states of the file system without interrupting service”. NILFS refers to these as checkpoints. In contrast, other file systems such as ZFS, can provide snapshots but they have to suspend operation to perform the snapshot operation. NILFS doesn’t have to do this. The snapshots (checkpoints) are part of the file system design itself.
I guess my questions boil down to:
1. Why is this approach fast on an SSD? Wouldn't the performance boost be more noticeable on a spinning disk?
2. Shouldn't someone try to make an FS optimised for SSD characteristics? Have someone already done this?
Sort of. It makes writes sequential (and thus fast) and reads nearly random (and thus slow).
Why is this approach fast on an SSD? Wouldn't the performance boost be more noticeable on a spinning disk?
Old, crappy SSDs have very slow random writes (which is like seeking), so log structuring increases write performance by orders of magnitude. Since random and sequential reads are equally fast on SSDs, the randomized layout doesn't hurt read performance as it does on disks.
Shouldn't someone try to make an FS optimised for SSD characteristics?
If the characteristics are different for every SSD model, no. Recent SSDs are becoming insensitive to access pattern, so the correct "optimization" is probably to perform no optimizations other than trim.
They do mention in the article that it would be a bad idea to use NILFS on your root partition because of heavy traffic, so lets say I decide to use it on a volume where I store all my media (where I pretty much write once, read a lot, and never delete). Now, a) why would I use an SSD for this given that they're so much more expensive and b) wouldn't I get comparable performance using an HDD here since my files will be laid out pretty much sequentially given my workload? Doesn't the same apply to companies storing mounds of data? Where is the middle ground where I would want to use NILFS on an SSD? One where I a) won't be sending enough write traffic to my drive to wear out my SSD too quickly and b) will be reading a lot of random blocks?
What am I missing? I see the merits in using NILFS, but I just don't see the purpose in comparing its performance against other filesystems on SSD's.
Wasn't some work done on log-structured file systems done by DEC some time ago? I can't find a reference to it.
Didn't Ousterhout (who, amongst other things, worked on a log based file system) work there on sabbatical?
> The concept was developed by John Ousterhout of TCL fame and Fred Douglis.
(Of course these typically target raw NAND flash via MTD / UBI layer, so typically aren't directly suited to the SSD block device abstraction)
Oh, and the jokes about having NILFS on your hard drive are going to be so lame...
there's a node I'd like to fsck!
I guess that's mostly a non-issue, as on Linux you usually don't use a swap file but a whole volume. There is support for using files, but you only use it as a last resort. Let's face it, if you know how to set up NILFS2 then you're probably able to create a swap partition.
The days of "swap" are numbered. The future is a smart memory hierarchy that uses knowledge about size/speed tradeoffs to make smart decisions about where to store things, but that won't look much like "swap".
Even with SSDs, it has become very unusual for "the system is currently swapping" to mean anything other than "the system's performance characteristics just became completely unacceptable". I stopped using a swap partition somewhere around 512MB to 1GB, and now my laptop that I didn't even try to max out the memory on shipped with 4GB. I can't hardly even fill up the cached buffers in 4GB in any reasonable period of time.
(Please note the difference between "very unusual" and "never". In fact, I can cite a server in my care that benefits from using a bit of swap. But that server is the exception, not the rule.)
Further, there may be some performance benefit to swapping idle process memory to make room for more buffer cache.
It's true that for interactive use (especially a laptop with spun-down drives), you may be happy without swap. But I don't see the facility being mothballed.
NILFS or MILFS whats the difference? About 35 years ...awwwww
/me ducks and covers
Why not? It's just a filesystem with layout that's a bit different to most, users of the filesystem should be completely oblivious to it.
Swap just needs a file it can randomly read and write to; if a fs can't provide such random access, it's not just swap it'll be useless for.