Message-ID: <20080315232221.GA24227@1wt.eu>
Date:	Sun, 16 Mar 2008 00:22:21 +0100
From:	Willy Tarreau <w@....eu>
To:	Daniel Phillips <phillips@...nq.net>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	David Newall <davidn@...idnewall.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [ANNOUNCE] Ramback: faster than a speeding bullet

On Sat, Mar 15, 2008 at 02:33:09PM -0800, Daniel Phillips wrote:
> On Saturday 15 March 2008 14:54, Willy Tarreau wrote:
> > I think it could get major adoption with ordered writes.
> 
> It already has ordered write when it is in flush mode.

but IIRC it works in write-through then.

> OK, I hear you. There will be an ordered write mode that uses barriers
> to decide the ordering.  It will greatly reduce the speed at which
> ramback can flush dirty data because of the need to wait synchronously
> on every barrier, of which there are many.  And thus will widen out the
> window during which UPS power must remain available if power goes out,
> in order to get all acknowledged transactions on to stable media.  The
> advantage is, the stable media always has a point-in-time version of
> the filesystem.
> 
> Don't expect this mode in the immediate future though, there are bugs
> to fix in the current driver, which already implements the required
> performance and stability requirements for a broad range of users.

ah, good!

> > > That is why I keep recommending that a ramback setup be replicated or
> > > mirrored, which people in this thread keep glossing over.  When
> > > replicated or mirrored, you still get the microsecond-level transaction
> > > times, and you get the safety too.
> > 
> > I agree, but in this case, you should present it this way. You have been
> > insisting too much on the average PC's reliability, the fact that no kernel
> > ever crashed for you, etc... So you are demonstrating that your product is
> > good provided that everything goes perfectly. All people who have experienced
> > software or hardware problems in the past (ie mostly everyone here) will not
> > trust your code because it relies on pre-requisites they know they do not
> > have.
> 
> That would have been a miscommunication then.  I see arguments coming
> in that suggest embedded solutions, EMC for example, are inherently more
> reliable than a Linux based solution.  Well guess what?  Some of those
> embedded solutions already use Linux.

But their RAM does not depend on a lot of factors to remain valid and
usable, which is the problem with the common PC.

> Also, peecees are much more reliable than people give them credit for,
> especially if you harden up the obvious points of failure such as fans
> and spinning disks.

and PSU.

> Once you have your system all hardened up, then
> you _still_ better replicate your important data.  Perhaps I should not
> admit this, but I simply fail to do that on the machine from which I am
> posting right now, which also runs my web server and mail system.  That
> is because I would have to reboot it to install ddsnap so I can replicate
> properly, and because the thing is so darn reliable that I just have
> not gotten around to it.  I do copy off the important files from time
> to time though, and do various other things to ameliorate the risk.  If
> it was enterprise data I would obviously do a lot more.

Securing every component simply reduces the risk of a loss of service.
What is important with data is to know the consequences of loss of service.
If that only means that no one can work and that the last second of work is
lost, it's generally acceptable. If it means everything is lost to a corrupted
FS, obviously it's not.

> > > There will be a whole bunch of patches from me that are SSD oriented,
> > > over time.  The fact is, enterprise scale ramdisks are here now, while
> > > enterprise scale flash is not.  Getting close, but not here.  And flash
> > > does not approach the write performance of RAM, not now and probably
> > > not ever.
> > 
> > My goal is not to replace RAM with flash, but disk with flash.
> 
> My immediate goal is to replace disk with RAM.

No, you're replacing disk activity with RAM activity. But you keep disk as
a backend, long-term storage.

> > You are 
> > against ordered writes for a performance reason. Use SSD instead of
> > hard drives and it will be as fast as sequential writes. Also, when
> > you say that enterprise scale flash is not there, I don't agree. You
> > can already afford hundreds of gigs of flash in 3,5" form factor. An
> > 1.6 TB SSD has even been presented at CES2008, with sales announced
> > for Q3. So clearly this will replace your hard drives soon, very soon.
> > Even if it costs $5k, that's a very acceptable solution to replace a
> > disk in a RAM-speed appliance.
> 
> Exactly what I mean: close but not there.  Those gigantic RAM boxes are
> shipping now, and the same company has got a 5 TB flash box coming down
> the pipe, and sooner than Q3.  But the RAM box will always outperform
> the flash box.  You just keep throwing writes at it until all available
> flash is in erase mode, and the thing slows down.  If that is not a
> problem for you, then great, you can also save a lot of money by going
> with flash.  But if nothing less than the ultimate in performance will
> do, RAM is the only way to go.

Sorry if I was not clear. I was not speaking about replacing the RAM with
flash, but only the disks. You keep the RAM for the speed, and use flash
for permanent storage instead of disks. No seek time, average RW speed now
slightly better than disks, that combined with your ramdisk and ordered
write-backs writes will have the best of both worlds : RAM speed and flash
reliability.

Willy

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