Merge pull request #34855 from dimitri-furman/dfurman/tf834

Clarify and update guidance for TF 834

Author: prmerger-automator[bot]

docs/t-sql/database-console-commands/dbcc-traceon-trace-flags-transact-sql.md

@@ -21,13 +21,14 @@ dev_langs:
 ms.custom:
   - build-2025
 ---
+
 # DBCC TRACEON - Trace Flags (Transact-SQL)
 
 [!INCLUDE [tsql-appliesto-ss-asdmi-md](../../includes/applies-to-version/sql-asdbmi.md)]
 
-Trace flags are used to set specific server characteristics or to alter a particular behavior. For example, Trace Flag 3226 is a commonly used startup trace flag that suppresses successful backup messages in the error log. Trace flags are frequently used to diagnose performance issues or to debug stored procedures or complex computer systems, but they might also be recommended by Microsoft Support to address behavior that is negatively affecting a specific workload.
+Trace flags are used to set specific server characteristics or to alter a particular behavior. For example, Trace Flag 3226 is a commonly used startup trace flag that suppresses successful backup messages in the error log. Trace flags might be used to diagnose or mitigate performance issues, but they might also be recommended by Microsoft to address behavior that is negatively affecting a specific workload. Conversely, Microsoft might recommend that you remove a trace flag if it causes or contributes to a problem impacting your workload.
 
-All documented trace flags and those recommended by Microsoft Support are fully supported in a production environment when used as directed. Trace flags in this list can have additional considerations regarding their particular usage, so it's advisable to carefully review all the recommendations given here and/or by your support engineer. Also, as with any configuration change in [!INCLUDE [ssNoVersion](../../includes/ssnoversion-md.md)], it's always best to thoroughly test the flag in a non-production environment before deploying.
+All documented trace flags and those recommended by Microsoft are fully supported in a production environment when used as directed. Trace flags in this list can have additional considerations regarding their particular usage, so it's advisable to carefully review all the recommendations given here and/or by your support engineer. Also, as with any configuration change in [!INCLUDE [ssNoVersion](../../includes/ssnoversion-md.md)], it's always best to thoroughly test the flag in a non-production environment before deploying.
 
 [!INCLUDE [entra-id](../../includes/entra-id.md)]
 
@@ -105,7 +106,7 @@ Trace flag behavior might not be supported in future releases of [!INCLUDE [ssNo
 | <a id="tf809"></a>**809** | Enables what is referred to as *Direct Write* behavior for the [hybrid buffer pool](../../database-engine/configure-windows/hybrid-buffer-pool.md). This mode requires that [persisted log buffer](../../relational-databases/databases/add-persisted-log-buffer.md) is enabled in the same database as hybrid buffer pool. *Direct Write* allows the hybrid buffer pool to serve as a cache for both dirty and clean pages, reducing the workload demands on the DRAM buffer pools for OLAP and OLTP style workloads. This trace flag was introduced in [!INCLUDE [sssql22-md](../../includes/sssql22-md.md)] and the behavior is enabled by default in [!INCLUDE [sssql22-md](../../includes/sssql22-md.md)] CU 1. If you're using [!INCLUDE [sssql22-md](../../includes/sssql22-md.md)] CU 1 and later versions, the trace flag is ignored. |
 | <a id="tf818"></a>**818** | Enables additional I/O diagnostics to check for Lost Write or Stale Read conditions during file I/O operations. Trace Flag 818 enables an in-memory ring buffer that is used for tracking the last 2,048 successful write operations that are performed by [!INCLUDE [ssNoVersion](../../includes/ssnoversion-md.md)], not including sort and workfile I/Os. When errors such as Error 605, 823, or 3448 occur, the incoming buffer's log sequence number (LSN) value is compared to the recent write list. If the LSN that is retrieved is older than the one specified during the write operation, a new error message is logged in the [!INCLUDE [ssNoVersion](../../includes/ssnoversion-md.md)] error log. For more information, see [SQL Server diagnostics detects unreported I/O problems due to stale reads or lost writes](/troubleshoot/sql/database-engine/database-file-operations/diagnostics-for-unreported-io-problems).<br /><br />**Note:** Starting with [!INCLUDE [ssSQL17](../../includes/sssql17-md.md)], this trace flag has no effect.<br /><br />**Scope:** Global only. |
 | <a id="tf830"></a>**830** | Disables detection and reporting of I/O requests that take a very long time to complete. By default [!INCLUDE [ssNoVersion](../../includes/ssnoversion-md.md)] uses a mechanism to detect read and write I/O requests that take a long time (typically longer than 15 seconds). This trace flag disables this detection. For more information, see [MSSQLSERVER_833](../../relational-databases/errors-events/mssqlserver-833-database-engine-error.md).<br /><br />**Note:** It isn't recommended that you enable this trace flag because you could decrease your ability to detect I/O issues on the system.<br /><br />**Scope:** Global only. |
-| <a id="tf834"></a>**834** | Uses large-page allocations for the buffer pool, columnstore, and in-memory tables. The large page size varies depending on the hardware platform, but the page size can be from 2 MB to 16 MB. Large pages are allocated at startup and are kept throughout the lifetime of the process. Trace Flag 834 improves performance by increasing the efficiency of the translation look-aside buffer (TLB) in the CPU. In other words, Trace Flag 834 increases the efficiency of managing physical to virtual memory address translation that is performed by memory management hardware. For information on [!INCLUDE [ssnoversion-md](../../includes/ssnoversion-md.md)] and Large Pages, review the following [blog](https://techcommunity.microsoft.com/blog/sqlserversupport/sql-server-and-large-pages-explained8230-/315787).<br /><br />**Note:** Trace Flag 834 applies only to 64-bit versions of [!INCLUDE [ssnoversion-md](../../includes/ssnoversion-md.md)]. You must have the Lock pages in memory user right to turn on Trace Flag 834. You can turn on Trace Flag 834 only at startup. Trace Flag 834 can prevent the server from starting if memory is fragmented and if large pages can't be allocated. Therefore, Trace Flag 834 is best suited for servers that are dedicated to [!INCLUDE [ssnoversion-md](../../includes/ssnoversion-md.md)].<br /><br />**Note:** When enabled, the large-page memory model preallocates all SQLOS memory at instance startup and doesn't return that memory to the OS.<br /><br />**Note:** If you're using the Columnstore Index feature of [!INCLUDE [ssSQL11](../../includes/sssql11-md.md)] and later versions, we don't recommend turning on Trace Flag 834. For more information, see [Interoperability issues between batch mode processing and large page memory model](/troubleshoot/sql/database-engine/performance/batch-mode-with-large-page-memory-issues). If using [!INCLUDE [sql-server-2019](../../includes/sssql19-md.md)] and columnstore, see Trace Flag [876](#tf876) instead.<br /><br />**Scope:** Global only. |
+| <a id="tf834"></a>**834** | Uses large-page allocations for all memory allocations within the SQL Server Operating System (SQLOS) workspace. The large page size varies depending on the hardware platform and can be from 2 MB to 16 MB. Large pages are allocated at startup and are kept throughout the lifetime of the process.<br /><br />In a certain narrow set of scenarios, Trace Flag 834 might improve performance by increasing the efficiency of the translation look-aside buffer (TLB) in the CPU. In other words, Trace Flag 834 increases the efficiency of managing physical to virtual memory address translation that is performed by the memory management hardware.<br /><br />Before you enable Trace Flag 834, follow the recommendations for maximum server memory configuration in [Server memory configuration options](../../database-engine/configure-windows/server-memory-server-configuration-options.md).<br /><br />**Note:** Trace Flag 834 applies only to 64-bit versions of [!INCLUDE [ssnoversion-md](../../includes/ssnoversion-md.md)]. You must have the Lock pages in memory user right to turn on Trace Flag 834. You can turn on Trace Flag 834 only at startup. Trace Flag 834 can prevent the server from starting if memory is fragmented and if large pages can't be allocated. Therefore, Trace Flag 834 is best suited for servers that are dedicated to [!INCLUDE [ssnoversion-md](../../includes/ssnoversion-md.md)].<br /><br />**Note:** When enabled, the large-page memory model preallocates all SQLOS memory at instance startup and doesn't return that memory to the OS.<br /><br />**Warning:** We don't recommend turning on Trace Flag 834 unless you thoroughly test it and determine that it materially benefits your workload. Enabling this trace flag can substantially increase the kernel CPU time and reduce performance. This occurs if memory becomes fragmented, requiring [!INCLUDE [ssNoVersion](../../includes/ssnoversion-md.md)] to call Windows APIs to allocate and deallocate memory instead of reusing memory cached in the SQLOS workspace. Trace Flag 834 can also cause unnecessary memory consumption because the unused part of committed memory blocks isn't released to the operating system.<br /><br />**Note:** If you're using columnstore indexes, don't enable Trace Flag 834. For more information, see [Interoperability issues between batch mode processing and large page memory model](/troubleshoot/sql/database-engine/performance/batch-mode-with-large-page-memory-issues). Instead, if using [!INCLUDE [sql-server-2019](../../includes/sssql19-md.md)] and later versions, consider using Trace Flag [876](#tf876) instead.<br /><br />**Scope:** Global only. |
 | <a id="tf836"></a>**836** | Use the max server memory option for the buffer pool. Trace Flag 836 causes [!INCLUDE [ssnoversion-md](../../includes/ssnoversion-md.md)] to size the buffer pool at startup based on the value of the max server memory option instead of based on the total physical memory. You can use Trace Flag 836 to reduce the number of buffer descriptors that are allocated at startup in 32-bit Address Windowing Extensions (AWE) mode.<br /><br />**Note:** Trace Flag 836 applies only to 32-bit versions of [!INCLUDE [ssnoversion-md](../../includes/ssnoversion-md.md)] that have the AWE allocation enabled. You can turn on Trace Flag 836 only at startup.<br /><br />**Scope:** Global only. |
 | <a id="tf845"></a>**845** | Enables locked pages on Standard SKUs of [!INCLUDE [ssNoVersion](../../includes/ssnoversion-md.md)], when the service account for [!INCLUDE [ssNoVersion](../../includes/ssnoversion-md.md)] has the Lock Pages in Memory privilege enabled. For more information, see [KB970070](https://support.microsoft.com/kb/970070) and [Server Memory Server Configuration Options](../../database-engine/configure-windows/server-memory-server-configuration-options.md#lock-pages-in-memory-lpim).<br /><br />**Note:** Starting with [!INCLUDE [ssSQL11](../../includes/sssql11-md.md)], this behavior is enabled by default for Standard SKUs, and Trace Flag 845 must not be used.<br /><br />**Scope:** Global only. |
 | <a id="tf876"></a>**876** | Uses large-page allocations for columnstore.<br /><br />**Note:** Unlike Trace Flag 834, using Trace Flag 876 doesn't preallocate SQLOS memory at instance startup, and unused memory can be released.<br /><br />**Applies to:** [!INCLUDE [sql-server-2019](../../includes/sssql19-md.md)] and later versions.<br /><br />**Note:** Ensure that you thoroughly test this option, before rolling it into a production environment.<br /><br />**Scope:** Global only. |