Re: [gentoo-user] ZFS

[covici@ccs.covici.com]

Pandu Poluan <pandu@poluan.info> wrote:

> On Tue, Sep 17, 2013 at 2:20 PM, Grant <emailgrant@gmail.com> wrote:
> > I'm convinced I need 3-disk RAID1 so I can lose 2 drives and keep
> > running.  I'd also like to stripe for performance, resulting in
> > RAID10.  It sounds like most hardware controllers do not support
> > 6-disk RAID10 so ZFS looks very interesting.
> >
> > Can I operate ZFS RAID without a hardware RAID controller?
> >
> 
> Yes. In fact, that's ZFS' preferred mode of operation (i.e., it
> handles all redundancy by itself).
> 
> > From a RAID perspective only, is ZFS a better choice than conventional
> > software RAID?
> >
> 
> Yes.
> 
> ZFS checksummed all blocks during writes, and verifies those checksums
> during read.
> 
> It is possible to have 2 bits flipped at the same time among 2 hard
> disks. In such case, the RAID controller will never see the bitflips.
> But ZFS will see it.
> 
> > ZFS seems to have many excellent features and I'd like to ease into
> > them slowly (like an old man into a nice warm bath).  Does ZFS allow
> > you to set up additional features later (e.g. snapshots, encryption,
> > deduplication, compression) or is some forethought required when first
> > making the filesystem?
> >
> 
> Snapshots is built-in from the beginning. All you have to do is create
> one when you want it.
> 
> Deduplication can be turned on and off at will -- but be warned: You
> need HUGE amount of RAM.
> 
> Compression can be turned on and off at will. Previously-compressed
> data won't become uncompressed unless you modify them.
> 
> > It looks like there are comprehensive ZFS Gentoo docs
> > (http://wiki.gentoo.org/wiki/ZFS) but can anyone tell me from the real
> > world about how much extra difficulty/complexity is added to
> > installation and ongoing administration when choosing ZFS over ext4?
> >
> 
> Very very minimal. So minimal, in fact, that if you don't plan to use
> ZFS as a root filesystem, it's laughably simple. You don't even have
> to edit /etc/fstab
> 
> > Performance doesn't seem to be one of ZFS's strong points.  Is it
> > considered suitable for a high-performance server?
> >
> > http://www.phoronix.com/scan.php?page=news_item&px=MTM1NTA
> >
> 
> Several points:
> 
> 1. The added steps of checksumming (and verifying the checksums)
> *will* give a performance penalty.
> 
> 2. When comparing performance of 1 (one) drive, of course ZFS will
> lose. But when you build a ZFS pool out of 3 pairs of mirrored drives,
> throughput will increase significantly as ZFS has the ability to do
> 'load-balancing' among mirror-pairs (or, in ZFS parlance, "mirrored
> vdevs")
> 
> Go directly to this post:
> http://phoronix.com/forums/showthread.php?79922-Benchmarks-Of-The-New-ZFS-On-Linux-EXT4-Wins&p=326838#post326838
> 
> Notice how ZFS won against ext4 in 8 scenarios out of 9. (The only
> scenario where ZFS lost is in the single-client RAID-1 scenario)
> 
> > Besides performance, are there any drawbacks to ZFS compared to ext4?
> >
> 
> 1. You need a huge amount of RAM to let ZFS do its magic. But RAM is
> cheap nowadays. Data... possibly priceless.
> 
> 2. Be careful when using ZFS on a server on which processes rapidly
> spawn and terminate. ZFS doesn't like memory fragmentation.
> 
> For point #2, I can give you a real-life example:
> 
> My mail server, for some reasons, choke if too many TLS errors happen.
> So, I placed "Perdition" in to capture all POP3 connections and
> 'un-TLS' them. Perdition spawns a new process for *every* connection.
> My mail server has 2000 users, I regularly see more than 100 Perdition
> child processes. Many very ephemeral (i.e., existing for less than 5
> seconds). The RAM is undoubtedly *extremely* fragmented. ZFS cries
> murder when it cannot allocate a contiguous SLAB of memory to increase
> its ARC Cache.
> 
> OTOH, on another very busy server (mail archiving server using
> MailArchiva, handling 2000+ emails per hour), ZFS run flawlessly. No
> incident _at_all_. Undoubtedly because MailArchiva use one single huge
> process (Java-based) to handle all transactions, so no RAM
> fragmentation here.
Spo do I need that overlay at all, or just emerge zfs and its module?
Also, I now have lvm volumes, including root, but not boot, how to
convert and do I have to do anything to my initramfs?

-- 
Your life is like a penny.  You're going to lose it.  The question is:
How do
you spend it?

         John Covici
         covici@ccs.covici.com