![]() ![]() ![]() If you have to wait for data to be written to a slow array of disks and get a response back, it can feel like timing storage with a sundial. The price of this safety is often performance, especially when writing to slow arrays of hard drives. Synchronous writes (also known as sync writes), are safer because the client system waits for the acknowledgment before it continues on. ZFS ZIL SLOG Selected Sync Write Scenarios On the server side, this is not a desirable scenario for anyone who wants to keep a job. If you look at client systems such as laptops, it is not uncommon to see asynchronous writes and data loss or corruption. your check bounces.) Modern systems are generally reliable, but asynchronous writes can cause issues. If you think about writing a check thinking you have money in the bank, and you do not, bad things happen (e.g. On the data security side of things, asynchronous writes are generally considered less “safe” because a disruption in the storage or network can mean that the system doing the write thinks that the data is written and safe on persistent storage when in fact it is not. With the asynchronous write, a system will send data and then continue to do the next task before it receives the acknowledge. At its essence, a system will send data to storage to be written and with synchronous write, it will wait until it receives the acknowledge from target storage. Synchronous and asynchronous writes may seem simple, yet the difference has a profound impact. ![]() That acknowledgment is important because it can have a dramatic impact on synchronous write performance Synchronous v. ![]() Until the client receives the acknowledgment, it does not know that data has been successfully received and is safe on storage. That acknowledge is an important step in the process. Here is the illustration: Basics Of Data Storage Finally, the storage server needs to acknowledge that data has been received. Once the data reaches the other side of the network at the storage server, that data needs to be received by the storage server. The transmission of the data from the client VM host. We essentially have three major operations that need to happen. That host needs to save data to its network storage so it can be accessed later by that VM or another VM host. Let us say that you have a VM running on a VM host. We are going to exclude all of the fun network stack bits, and impacts of technology like RDMA. We are going to use a simple example of a client machine, say a virtual machine host, writing to a ZFS storage server. The absolute basics are required to understand what the ZFS ZIL SLOG does. Instead, we wanted to provide a bit of background as to what happens when you write data, at least at a high-level. In the original draft of this article, we started with ZFS. Background: What happens when you write data to storage? If you want to help by coding new features for OpenZFS, this is not the ultra-technical guide you need. We are going to try keeping this at a high enough level so that a broad audience can understand what is going on. As we have hinted at STH, we just did a bunch of benchmarking on current options so we will have some data for you in follow-up pieces. We are then going to discuss what makes a good device and some common pitfalls to avoid when selecting a drive. In this article, we are going to discuss what the ZIL and SLOG are. The ZFS ZIL SLOG is essentially a fast persistent (or essentially persistent) write cache for ZFS storage. While reads can be easily cached in RAM, writes need to be cached on persistent storage to maintain the integrity of the storage. That is an important piece of background information because with disk-based arrays performance is always a concern due to the slow media. ZFS was born in an era disk-based storage. Today we are seeing many storage systems predicated on flash storage but ZFS was born of a different era. #Zfs raid level openzfs softwareZFS is the popular storage system that was born out of Sun Microsystems (now Oracle.) If you are looking for a piece of software that has both zealots for and against, ZFS should be at the top of your list. ![]()
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