3/27/2023 0 Comments Iflash database![]() If your disks contain “inactive, colder data” which doesn’t enter the cache, exactly how is that cache going to be efficiently populated? Keeping inactive data on Exadata’s disks is not only financially ruinous, it impacts the effect of having such an increased flash cache capacity. The flash cache, according to the sources in the first table above, holds between 88TB and 440TB of data – but, since it’s a cache, that data must be read from a persistent source somewhere. What about the “inactive, colder data” that remains solely on disk? Well ok… let’s think about that for a minute. Three copies on disk and one on flash? That doesn’t seem like the most efficient way to utilise what is, after all, extremely expensive storage. And because ASM mirroring is required, it will actually be on disk twice – or, if you need zero-downtime upgrades, three times. Yet in Exadata flash is only ever used as a cache, so the database in question is also going to be located on disk. Yes, that’s right: on Exadata X4, your entire database is now likely to be in flash. Except you still have to pay for the disks… The All Flash Database Machineīefore we go any further, here’s a quote from Oracle’s Vice President of Product Management, Tim Shetler, discussing the increased flash capacity in Exadata X4: Oracle has taken the decision, almost by stealth, to make the Exadata X4 into an all-flash database machine. Which is where I finally get to my point. What is the point of having less disk capacity than you have flash cache capacity? Clue: there is no point. triple mirroring) which means even less capacity. If you want to perform zero-downtime upgrades then you need “high redundancy” (i.e. double mirroring) the usable disk capacity with HP disks is just 90TB, according to the datasheet. Remember that Oracle claims the Exadata Smart Flash Cache can now contain 88TB of data? But if all data on disk must be mirrored, then with ASM “normal redundancy” (i.e. ![]() ![]() the use of flash as cache only (meaning all data in flash is eventually destaged to disk) rather than a tier of storage.the reliance on ASM software mirroring (meaning all data is stored either twice or three times), and.The answer lies in two of Oracle’s fundamental design choices for the Exadata architecture: Why on Earth would Oracle make that change? If customers wanted more capacity, couldn’t they just buy the storage expansion racks? Design Dilemmas That’s an extra 50% average rotational latency. The average rotational latency for a 10k RPM drive is 6ms. How much slower are we talking? Well, the average rotational latency of a 15k RPM drive is 4ms. In Exadata X4, the high performance disks are slower than in Exadata X3. That’s not the important bit though, it’s the sacrifice that Oracle had to make to do this: 10k RPM disk drives instead of 15k RPM. See if you can spot the most important change to be made since the introduction of flash back in the Sun Oracle v2 (second generation) machine:ĭid you notice? In the X4 model storage cells, the HP disks have now doubled in capacity. It’s fair to say that the majority of customers buy the high performance version ( * see comments below) – after all, Exadata is a very expensive solution aimed at solving performance problems, so performance is generally high up on a customer’s list of requirements. The disks can be bought in two options: high performance or high capacity (known as HP and HC respectively). Each full rack contains 14 storage cells, with each cell containing 12 rotating magnetic disks as well as four PCIe flash cards ( made by LSI and then rebranded as Sun). the Exadata v2), still uses the hybrid approach in Exadata. Oracle, despite being one of the early adopters of flash with its Sun Oracle Database Machine (i.e. Most storage vendors now offer all-flash arrays as part of their product lineup, although one or two still insist on the hybrid approach where data is located on disk but flash is used as a tiering or caching layer to improve performance. Right now, in the storage industry, there is a paradigm shift taking place as primary data moves from rusty old spinning disks to semiconductor-based NAND flash storage. And it’s the change that I think gives the biggest insight into Oracle’s thinking. This second part of the article will look at the implications of these changes, but to make things more interesting there’s one specific change I haven’t mentioned until now. Noticeably absent from the press release and datasheet is the information that this new feature apparently requires the Advanced Compression license, potentially adding over $1m to the list price of a full rack (see slide 22 of this Oracle presentation). The source of this confusion appears to be the claim that a new feature called Exadata Smart Flash Cache Compression will allow more data to fit into flash. ![]() “flash cache compression expands capacity to 88TB ( raw)” ![]()
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