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Delphix Overview

Update: Here’s the link to the recording of the webinar

I’ll b online tomorrow morning (Friday 5th, 9:00 Pacific time, 5:00 pm UK) in a webinar with Kyle Hailey to talk about my first impressions of Delphix, so I thought I’d write up a few notes beforehand.

I’ve actually installed a complete working environment on my laptop to model a production setup. This means I’ve got three virtual machines running under VMWare: my “production” machine (running Oracle on OEL 5, 64-bit), a “development” machine (which has the software installed, again on OEL 5, 64-bit), and a machine which I specified as Open Solaris 10, 64-bit for the Delphix server VM (pre-release bloggers’ version). The two Linux servers are running with 2.5GB of RAM, the Delphix server is running with 8GB RAM, and all three machines are running 2 virtual CPUs. (My laptop has an Intel quad core i7, running two threads per CPU, 16GB RAM, and 2 drives of 500GB each.) The Linux machines were simply clones of another virtual machine I previously prepared and the purpose of the exercise was simply to see how easy it would be to “wheel in” a Delphix server and stick it in the middle. The answer is: “pretty simple”. (At some stage I’ll be writing up a few notes about some of the experiments I’ve done on that setup.)

To get things working I had to create a couple of UNIX accounts for a “delphix” user on the Linux machines, install some software, give a few O/S privileges to the user (mainly to allow it to read and write a couple of Oracle directories), and a few Oracle privileges. The required Oracle privileges vary slightly with the version of Oracle and your prefered method of operation, but basically the delphix user needs to be able to run rman, execute a couple of Oracle packages, and query some of the dynamic performance views. I didn’t have any difficulty with the setup, and didn’t see any threats in the privileges that I had to give to the delphix user. The last step was simply to configure the Delphix server to give it some information about the Linux machines and accounts that it was going to have access to.

The key features about the Delphix server are that it uses a custom file system (DxFS, which is based on ZFS with a number of extensions and enhancements) and it exposes files to client machines through NFS; and there are two major components to the software that make the whole Delphix package very clever.

Oracle-related mechanisms

At the Oracle level, the Delphix server sends calls to the production database server to take rman backups (initially a full backup, then incremental backups “from SCN”); between backup requests it also pulls the archived redo logs from the production server – or can even be configured to copy the latest entries from the online redo logs a few seconds after they’ve been written (which is one of the reasons for requiring privileges to query some of the dynamic performance views, but the feature does depend on the Oracle version).

If you want to make a copy of the database available, you can use the GUI interface on the Delphix server to pick a target machine, invent a SID, and Service name, and pick an SCN (or approximate timetamp) that you want to database to start from, and within a few minutes the Delphix server will have combined all the necessary backup pieces, applied any relevant redo, and configured your target machine to start up an instance that can use the (NFS-mounted) database that now exists on the Delphix server. I’ll explain in a little while why this is a lot cleverer than a simple rman “restore and recover”.


Supporting the Oracle-related features, the other key component of the Delphix server is the Delphix file-system (DxFS). I wrote a little note a few days ago to describe how Oracle can handle “partial” updates to LOB values – the LOB exists in chunks with an index on (lob_id, chunk_number) that allows you to pick the right chunks in order. When you update a chunk in the LOB Oracle doesn’t really update the chunk, it creates a new chunk and modifies the index to point at it. If another session has a query running that should see the old chunk, though, Oracle can read the index “as at SCN” (i.e. it creates a read consistent copy of the required index blocks) and the read-consistent index will automatically be pointing at the correct version of the LOB chunk. DxFS does the same sort of thing – when a user “modifies” a file system block DxFS doesn’t overwrite the original copy, it writes a new copy to wherever there’s some free space and maintains some “indexing” metadata that tells it where all the pieces are. But if you never tell the file system to release the old block you can ask to see the file as at a previous point in time at no extra cost!

But DxFs is even cleverer than that because (in a strange imitation of the “many worlds” interpretation of quantum theory) a single file can have many different futures. Different users can be identified as working in different “contexts” and the context is part of the metadata describing the location of blocks that belong to the file. Imagine we have a file with 10 blocks sitting on DxFs - in your context you modify blocks 1,2 and 3 but at the same time I modify blocks 1,2 and 3 in my context. Under DxFS there are now 16 blocks associated with that file – the original 10, your three modified blocks and my three modified blocks and, depending on timestamp and context, someone else could ask to see any one of three different versions of that file – the original version, your version, or my version.

Now think of that in an Oracle context. If we copy an entire set of database files onto DxFS, then NFS-mount the files on a machine with Oracle installed, we can configure and start up an instance to use those files. At the same time we could NFS-mount the files on another machine, configuring and starting another instance to use the same data files at the same time! Any blocks changed by the first instance would be written to disc as private copies, any blocks changed by the second instance would be written to discs as private copies – if both instances managed to change 1% of the data in the course of the day then DxFs would end up holding 102% of the starting volume of data: the original datafiles plus the two sets changed blocks – but each instance would think it was the sole user of its version of the files.

There’s another nice (database-oriented) feature to Delphix, though. The file system has built-in compression that operates at the “block” level. You can specify what you mean by the block size (and for many Oracle sites that would be 8KB) and the file system would transparently apply a data compression algorithm on that block boundary. So when the database writer writes an 8KB block to disc, the actual disc space used might be significantly less than 8KB, perhaps by a factor of 2 to 3. So in my previous example, not only could you get two test databases for the space of 1 and a bit – you might get two test databases for the space of 40% or less of the original database.

Delphix vs. rman

I suggested earlier on that Delphix can be a lot clever than an rman restore and recover. If you take a full backup to Delphix on Sunday, and a daily incremental backup (let’s preted that’s 1% of the database per day) for the week, then Delphix can superimpose each incremental onto the full backup as it arrives. So on Monday we construct the equivalent of a full Monday backup, on Tuesday we construct the equivalent of a full Tuesday backup, and so on. But since DxFS keeps all the old copies of blocks this means two things that we can point an instance at a full backup for ANY day of the week simply by passing a suitable “timestamp” to DxFs – and we’ve 7 full backups for the space of 107% of a single full backup.

There are lots more to say, but I think they will have to wait for tomorrow’s conversation with Kyle, and for a couple more articles.

Register of Interests / Disclosure

Delphix Corp. paid my consultancy rates and expenses for a visit to the office in Menlo Park to review their product.