-----Original Message-----
From: Joseph Glanville [mailto:joseph.glanville-2MxvZkOi9dvvnOemgxGiVw@public.gmane.org]
Sent: Saturday, February 09, 2013 1:10 AM
To: Amit Kale
Cc: Kent Overstreet; Michel Lespinasse; Darrick J. Wong; linux-bcache;
device-mapper development; Kent Overstreet; Mike Snitzer; LKML; Jason
Warr; thornber-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org
Subject: Re: [RFC] [DONOTAPPLY] [PATCH] enhanceio: STEC EnhanceIO SSD
caching software for Linux kernel

On 7 February 2013 12:54, Amit Kale [off-list ref] wrote:

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-----Original Message-----
From: Kent Overstreet [mailto:koverstreet-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org]
Sent: Wednesday, February 06, 2013 4:01 PM
To: Amit Kale
Cc: Michel Lespinasse; Darrick J. Wong; linux-bcache; device-mapper
development; Kent Overstreet; Mike Snitzer; LKML; Jason Warr;
thornber-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org
Subject: Re: [RFC] [DONOTAPPLY] [PATCH] enhanceio: STEC EnhanceIO

SSD

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caching software for Linux kernel

On Thu, Feb 07, 2013 at 06:57:40AM +0800, Amit Kale wrote:

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-----Original Message-----
From: Michel Lespinasse [mailto:walken-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org]
Sent: Friday, February 01, 2013 4:58 PM
To: Darrick J. Wong
Cc: Amit Kale; linux-bcache; device-mapper development; Kent
Overstreet; Mike Snitzer; LKML; Jason Warr; thornber-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org
Subject: Re: [RFC] [DONOTAPPLY] [PATCH] enhanceio: STEC

EnhanceIO

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SSD caching software for Linux kernel

On Fri, Feb 1, 2013 at 4:44 PM, Darrick J. Wong
[off-list ref] wrote:

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This is a patch to migrate STEC's enhanceio driver out of

their

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github

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repository and into the staging tree.  From their README:

"EnhanceIO driver is based on EnhanceIO SSD caching software
product developed by STEC Inc. EnhanceIO was derived from
Facebook's open source Flashcache project. EnhanceIO uses SSDs
as cache devices for traditional rotating hard disk drives
(referred to as source volumes

throughout this document).

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EnhanceIO can work with any block device, be it an entire

physical

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disk, an individual disk partition,  a RAIDed DAS device, a

SAN

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volume, a device mapper volume or a software RAID (md)

device."

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What's your take on the benefits of this vs bcache ?

EnhanceIO was designed for and has been validated in enterprise
environments. The important benefits are - 1. There is no downtime

for cache creation, deletion, editing properties,
writeback/readonly/writethrough mode change.

True of bcache.

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2. Wb mode comes with an option to control whether dirty data
should

be clean-up across reboots, which prevents SSD/HDD going out of

sync.

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No idea why you'd want this. You have to be able to handle a
rebooting with a dirty cache, if you hope to handle unclean shutdown
- bcache does this. And if you're running in writeback mode there's
probably going to be many gigabytes of dirty data in your cache, at
least sometimes, and you aren't going to want to wait for that to
flush before you reboot.

If you want to flush dirty data with bcache, you can switch back to
writethrough mode whenever you want.

We have two modes of operation with writeback

1. Warm restart mode (default) - Reboots are quick. Dirty data is

kept as it is across reboots. So is clean data. Abrupt reboots are
handled correctly by throwing away clean data and retaining dirty data.
The throwing away clean data is a fallout of an optimization done to
skip metadata updates for clean data.

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2. Cold restart mode - A reboot or a shutdown involves clean-up of

dirty data. This is for paranoid sysads who are afraid that iSCSI HDD
setup may cause them to be reattached. Switching to writethrough mode
will achieve the same thing, however with a persistent cache
configuration in place, the cache will be write-through after a reboot.

How does Enhance-IO prevent mounting of the backing device if the cache
device doesn't show up due to failure etc?
I was reviewing the setup/tear-down code but I couldn't find where EIO
writes any superblock etc to the backing device to prevent udev etc
from mounting the filesystem.

EnhanceIO kernel driver doesn't do anything to prevent it. User level scripts have to do that. Kindly check the documentation for an example of how to do this using udev. We have tested several failure scenarios and udev scripts to handle them correctly.

-Amit

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I am happy to see bcache being compliant with the rest of the items I

had noted.

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-Amit

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3. Our in-house testing was done for large setups with 500GB+SSDs
and

proportionately large HDDs, on 24CPU machines with plenty of RAM.
It's survived heavy IO loads without any locking or corruption

problems.

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True of bcache.

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4. Error handling is exactly what enterprises look for -

writethrough/readonly modes work seamlessly regardless of SSD

failures.

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In all the three caching modes, the guarantees of completion in
presence of IO errors or shutdowns, in terms of granularity and
persistence of data written, is identical to underlying HDDs.

True of bcache.

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5. It works for all known block devices - Software RAIDs, full
block

devices with or without partitions, individual partitions, various
intelligent block devices.

True of bcache.

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-Amit

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Which of the above information was proprietary, and should I not be
resending this to lkml or - who?

...

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This electronic transmission, and any documents attached hereto, may contain confidential, proprietary and/or legally privileged information. The information is intended only for use by the recipient named above. If you received this electronic message in error, please notify the sender and delete the electronic message. Any disclosure, copying, distribution, or use of the contents of information received in error is strictly prohibited, and violators will be pursued legally.