generate qr code c# .net Determining the required number of disks and controllers in Visual C#.NET

Generation QR-Code in Visual C#.NET Determining the required number of disks and controllers

Determining the required number of disks and controllers
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directly attached to a server. It s increasingly common for organizations to use a mixture of both SCSI drives for performance-sensitive applications and SATA drives for applications requiring high amounts of storage. An example of this for a database application is to use SCSI drives for storing the database and SATA drives for storing online disk backups. SAS Serial Attached SCSI (SAS) disks connect directly to a SAS port, unlike traditional SCSI disks, which share a common bus. Borrowing from aspects of Fibre Channel technology, SAS was designed to break past the current performance barrier of the existing Ultra320 SCSI technology, and offers numerous advantages owing to its smaller form factor and backward compatibility with SATA disks. As a result, SAS drives are growing in popularity as an alternative to SCSI.
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Fibre Channel Fibre Channel allows high-speed, serial duplex communications between storage systems and server hosts. Typically found on SANs, Fibre Channel offers more flexibility than a SCSI bus, with support for more physical disks, more connected servers, and longer cable lengths. Solid-state disks Used today primarily in laptops and consumer devices, solid-state disks (SSDs) are gaining momentum in the desktop and server space. As the name suggests, SSDs use solid-state memory to persist data in contrast to rotating platters in a conventional hard disk. With no moving parts, SSDs are more robust and promise near-zero seek time, high performance, and low power consumption. We ll cover SSDs in more depth later in this chapter.
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Bus bandwidth
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When designing a storage system with many physical disks to support a large number of reads and writes, we must consider the ability of the I/O bus to handle the throughput. As you learned in the previous section, typical OLTP applications consist of random I/O with a moderate percentage of disk time seeking data, with disk latency (the time between disk request and response) an important factor. In contrast, OLAP applications spend a much higher percentage of time performing sequential I/O thus the throughput is greater and bandwidth requirements are higher. In a direct-attached SCSI disk enclosure, the typical bus used today is Ultra320, with a maximum throughput of 320MB/second per channel. Alternatively, a 2 Gigabit Fibre Channel system offers approximately 400MB/second throughput in full duplex mode. In our example of 2,000 disk transfers per second, assuming these were for an OLTP application with random I/O and 8K I/O transfers (the SQL Server transfer size for random I/O), the bandwidth requirements can be calculated as 2,000 times 8K, which is a total of 16MB/second, well within the capabilities of either Ultra320 SCSI or 2 Gigabit Fibre Channel. Should the bandwidth requirements exceed the maximum throughput, additional disk controllers and/or channels will be required to support the load. OLAP
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Storage system sizing
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applications typically have much higher throughput requirements, and therefore have a lower disk to bus ratio, which means more controllers/channels for the same number of disks. You ll note that we haven t addressed storage capacity requirements yet. This is a deliberate decision to ensure the storage system is designed for throughput and performance as the highest priority.
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A note on capacity
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A common mistake made when designing storage for SQL Server databases is to base the design on capacity requirements alone. A guiding principle in designing high-performance storage solutions for SQL Server is to stripe data across a large number of dedicated disks and multiple controllers. The resultant performance is much greater than what you d achieve with fewer, higher-capacity disks. Storage solutions designed in this manner usually exceed the capacity requirements as a consequence of the performance-centric approach. In our previous example (where we calculated the need for 16 disks), assuming we use 73GB disks, we have a total available capacity of 1.1TB. Usable space, after RAID 10 is implemented, would come down to around 500GB. If the projected capacity requirements for our database only total 50GB, then so be it. We end up with 10 percent storage utilization as a consequence of a performancecentric design. In contrast, a design that was capacity-centric would probably choose a single 73GB disk, or two disks to provide redundancy. What are the consequences of this for our example Assuming 125 IOPS per disk, we d experience extreme disk bottlenecks with massive disk queues handling close to 2,000 required IOPS! While low utilization levels will probably be frowned upon, this is the price of performance, and a much better outcome than constantly dealing with disk bottlenecks. A quick look at any of the server specifications used in setting performance records for the Transaction Processing Performance Council (tpc.org) tests will confirm a lowutilization, high-disk-stripe approach like the one I described. Finally, placing capacity as a secondary priority behind performance doesn t mean we can ignore it. Sufficient work should be carried out to estimate both the initial and future storage requirements. Running out of disk space at 3 a.m. isn t something I recommend! In this section, I ve made a number of references to various RAID levels used to provide disk fault tolerance. In the next section, we ll take a closer look at the various RAID options and their pros and cons for use in a SQL Server environment.
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