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Re: [dm-devel] Announcement: STEC EnhanceIO SSD caching software for Linux kernel



Hi Joe, Kent,

[Adding Kent as well since bcache is mentioned below as one of the contenders for being integrated into mainline kernel.]

My understanding is that these three caching solutions all have three principle blocks.
1. A cache block lookup - This refers to finding out whether a block was cached or not and the location on SSD, if it was.
2. Block replacement policy - This refers to the algorithm for replacing a block when a new free block can't be found.
3. IO handling - This is about issuing IO requests to SSD and HDD.
4. Dirty data clean-up algorithm (for write-back only) - The dirty data clean-up algorithm decides when to write a dirty block in an SSD to its original location on HDD and executes the copy. 

When comparing the three solutions we need to consider these aspects.
1. User interface - This consists of commands used by users for creating, deleting, editing properties and recovering from error conditions.
2. Software interface - Where it interfaces to Linux kernel and applications.
3. Availability - What's the downtime when adding, deleting caches, making changes to cache configuration, conversion between cache modes, recovering after a crash, recovering from an error condition.
4. Security - Security holes, if any.
5. Portability - Which HDDs, SSDs, partitions, other block devices it works with.
6. Persistence of cache configuration - Once created does the cache configuration stay persistent across reboots. How are changes in device sequence or numbering handled.
7. Persistence of cached data - Does cached data remain across reboots/crashes/intermittent failures. Is the "sticky"ness of data configurable.
8. SSD life - Projected SSD life. Does the caching solution cause too much of write amplification leading to an early SSD failure.
9. Performance - Throughput is generally most important. Latency is also one more performance comparison point. Performance under different load classes can be measured.
10. ACID properties - Atomicity, Concurrency, Idempotent, Durability. Does the caching solution have these typical transactional database or filesystem properties. This includes avoiding torn-page problem amongst crash and failure scenarios.
11. Error conditions - Handling power failures, intermittent and permanent device failures.
12. Configuration parameters for tuning according to applications.

We'll soon document EnhanceIO behavior in context of these aspects. We'll appreciate if dm-cache and bcache is also documented.

When comparing performance there are three levels at which it can be measured
1. Architectural elements
1.1. Throughput for 100% cache hit case (in absence of dirty data clean-up)
1.2. Throughput for 0% cache hit case (in absence of dirty data clean-up)
1.3. Dirty data clean-up rate (in absence of IO)
2. Performance of architectural elements combined
2.1. Varying mix of read/write, sustained performance.
3. Application level testing - The more real-life like benchmark we work with, the better it is.

Thanks.
-Amit

> -----Original Message-----
> From: linux-kernel-owner vger kernel org [mailto:linux-kernel-
> owner vger kernel org] On Behalf Of thornber redhat com
> Sent: Wednesday, January 16, 2013 4:16 PM
> To: device-mapper development
> Cc: Mike Snitzer; LKML
> Subject: Re: [dm-devel] Announcement: STEC EnhanceIO SSD caching
> software for Linux kernel
> 
> Hi Amit,
> 
> I'll look through EnhanceIO this week.
> 
> There are several cache solutions out there; bcache, my dm-cache and
> EnhanceIO seeming to be the favourites.  In suspect none of them are
> without drawbacks, so I'd like to see if we can maybe work together.
> 
> I think the first thing we need to do is make it easy to compare the
> performance of these impls.
> 
> I'll create a branch in my github tree with all three caches in.  So
> it's easy to build a kernel with them.  (Mike's already combined dm-
> cache and bcache and done some preliminary testing).
> 
> We've got some small test scenarios in our test suite that we run [1].
> They certainly flatter dm-cache since it was developed using these.
> It would be really nice if you could describe and provide scripts for
> your test scenarios.  I'll integrate them with the test suite, and then
> I can have some confidence that I'm seeing EnhanceIO in its best light.
> 
> The 'transparent' cache issue is a valid one, but to be honest a bit
> orthogonal to cache.  Integrating dm more closely with the block layer
> such that a dm stack can replace any device has been discussed for
> years and I know Alasdair has done some preliminary design work on
> this.  Perhaps we can use your requirement to bump up the priority on
> this work.
> 
> On Tue, Jan 15, 2013 at 09:19:10PM +0800, Amit Kale wrote:
> > 5. We have designed our writeback architecture from scratch.
> > Coalescing/bunching together of metadata writes and cleanup is much
> > improved after redesigning of the EnhanceIO-SSD interface. The DM
> > interface would have been too restrictive for this. EnhanceIO uses
> set
> > level locking, which improves parallelism of IO, particularly for
> > writeback.
> 
> I sympathise with this; dm-cache would also like to see a higher level
> view of the io, rather than being given the ios to remap one by one.
> Let's start by working out how much of a benefit you've gained from
> this and then go from there.
> 
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> 
> Please do not use this signature when sending to dm-devel.  If there's
> proprietary information in the email you need to tell people up front
> so they can choose not to read it.
> 
> - Joe
> 
> 
>   [1] https://github.com/jthornber/thinp-test-
> suite/tree/master/tests/cache
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