I/O Rates and Write Frequency

As mentioned earlier, write frequency may be expressed in terms of transactions per day. This is a common practice when describing processing capabilities of OLTP applications. To apply this write frequency to our calculations above, we need to convert this into I/Os per day.

One way of approximating the number of I/Os from transactions per day of a typical OLTP application is to refer to Transaction Processing Council's (www.tpc.org) TPC-C benchmark specifications. TPC-C is a very popular benchmark tool for comparing OLTP performance on different hardware and software configurations. The unit of performance measurement of TPC-C is tpmC, which is basically TPC-C transactions per minute. TPC-C rule of thumb estimate each tpmC (in an optimum OLTP environment) to correspond to 0.5 I/Os.

1 tpmC	= 1 txn/min; 0.5 IOPS per tpmC
1 txn/min	= 0.5 IOPS
1 txn	= 0.5 IOPS X 1 min X 60 sec/1 min = 30 I/Os

Therefore, TPC-C rules of thumb on tpmC 0.5 IOPS per tpmC = 30 I/Os per transaction.

Using this derivation, we can translate 2,000,000 transactions a day to I/Os per day:

I/Os per day

= 2,000,000 transactions per day x 30 I/Os per transaction = 60,000,000 I/Os per day

The computation above covers all the I/Os per day whether they are read or write I/Os. For our purpose, we are only concerned with write I/Os and thus have to assume a percentage of this total as write I/Os.

Most benchmark data on OLTP applications show that 60-70% of total I/Os are reads. We can therefore estimate write I/Os as:

Write I/Os per day

= 40% of 60,000,000 I/Os per day = 24,000,000 write I/Os per day

RAID Factors

For this example, we will make use of 155 GB E-Disk^® Flash drives in an external RAID subsystem implementing RAID 5 with a minimum of five disks (2 TB of data will of course consist of at least 13 X 155 GB disks in a single RAID 5 array). There will be a couple of assumptions we will have to make with this RAID 5 implementation:

First, external subsystem write cache is also disabled. Similar to that of disabling write cache in an E-Disk^® Flash drive, all write I/Os sent by host to external RAID subsystem writes directly to disk and does not cache any updates to cache memory of RAID controllers.

RAID will stripe data across all disks in a RAID 5 array. Depending on RAID 5 implementation, a block of data sent to RAID controllers may write data anywhere from two disks (data and parity) to all the disks in an array (worst case). We will assume the worst case scenario such that any block of data sent to the array results in writes on all of the disks, thereby wearing out all disks at the same rate. Note though that this is really a worst-case scenario and will rarely ever happen, if at all. If, for example, RAID 5 implementation writes to only a third of the total number of disks, endurance numbers can easily triple simply because of this RAID factor alone.

I/O Block Size for OLTP Environments

Based on an earlier discussion, I/O block sizes for typical OLTP environments are small and may range anywhere from 512 Bytes to 8 KB. Since the physical block size in E-Disk^® Flash chips is 16 KB, any write I/O with a block size from 512 Bytes to 8 KB will correspond to only one write in a single physical block in an E-Disk^® drive.

Endurance in Days

To compute for endurance, we will simply have to identify the max. write I/Os for a 155 GB E-Disk^® drive and use write frequency in terms of I/Os per day.

Max. write cycles for 155 GB E-Disk^® drive = 9,961,472,000,000
(*computed earlier in previous section example)

Estimated Write Endurance (days) = 9,961,472,000,000 / 24,000,000 = 415,061 days
Estimated Write Endurance (years) = 415,061 days / (365 days/year) = 1,137 years

Price Trend

Now that we've established the technical viability of solid state flash disks in mission-critical database applications, let's take a look at the practicality of flash disk deployment in enterprise storage. One major thing going for flash solid state disks is the continued decline in flash memory quotes. The huge market for portable electronic devices such as mobile phones, digital cameras, and MP3 players has driven semiconductor manufacturers to boost flash memory density and output. Market research firm Web-Feet Research predicts that the price per MByte of solid state flash disk will fall by an average of 80.86 percent annually within a 5-year period starting 2004.

Figure 1: Disk drive comparison, $ / MB

The price gap between HDDs and solid state flash disks is seen to narrow significantly-with a difference of only about $0.05 per MByte in 2007-while flash solid state disks dramatically increase in performance over HDDs (at 100x-150x over in terms of sustained transfers and IOPS). Web-Feet Research also projects a major decline in the solid state flash disk/HDD cost ratio by approximately four times, from 433:1 ($0.078 vs. $0.0018 per MByte) in 2003 to 107:1 ($0.096 vs. $0.0009 per MByte) in 2006. Solid state flash disks will also maintain its cost-per-MByte advantage over DRAM-based SSDs.

Conclusion

Based on the foregoing discussions, we can conclude that flash solid state disks are good IT investments even in high write environments such as database applications. The sample case involving BiTMICRO's E-Disk flash solid state disk (Tables 2 to 6, Sample OLTP environment) are conservative estimates of actual attainable Erase/Write wear-out life. We have made several assumptions that drastically increase the number of writes on E-Disk^® Flash drives. For one, we have disabled the cache. Enabling the E-Disk® SSD cache would definitely extend endurance in Examples 1 to 2 and Tables 2 to 6 by several orders of magnitude. Even in our sample OLTP application environment, we also assumed the worst case RAID 5 scenario where all disks in a RAID array are written to, notwithstanding the reduction in writes to the array that RAID cache provides.

Despite these very conservative estimates, the endurance figures derived are actually very high. Also, if we were to include BiTMICRO's accelerated Erase/Write endurance verification and testing, E-Disk^® erase/write endurance can be more than 15 times the values in Tables 2 to 6 and Examples 1 and 2. All of these prove that despite the erase/write endurance limitations, Flash drives are reasonably priced and can outlast traditional storage devices for practical use in database and other enterprise applications.

About the Author
Francisco Fronda is currently the Product Manager for BiTMICRO Networks, Inc. Prior to joining BiTMICRO, he handled sales and technical solutions for IBM Storage Systems Group. You may contact him at 1 (510) 743-3170 or send an email at francisco.fronda@bitmicro.com.

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